Author:Imaam Muhammad Naasir-ud-Deen Al-Albaanee
1. 'Itikaaf (secluding oneself in the masjid) is a recommended act in Ramadaan as well as any other day in the year. The source for that is found in Allaah's saying: "...while you are making 'Itikaaf in the masaajid." And there are also many authentic ahaadeeth about the Prophet's (saws) 'Itikaaf and narrations from the Salaf about it also. They are mentioned in the Musannafs of Ibn Abee Shaybah and 'Abdur-Razzaaq.
It is authentically reported that the Prophet (saws) made 'Itikaaf in the last ten days of Shawaal, and that 'Umar (raa) said to the Prophet (saws): "I made an oath (to Allaah) in the Days of Ignorance that I would make 'Itikaaf for one night in the Masjid Al-Haraam, (should I do it)?" The Prophet (saws) responded: "Fulfill your oath." So he made 'Itikaaf for one night. 
2. Observing it in Ramadaan is established in the hadeeth of Abu Hurairah (raa): "Allaah's Messenger (saws) would make 'Itikaaf for ten days in every Ramadaan. But when it was the year in which he died, he made 'Itikaaf for twenty days." 
3. The best time to do it is in the last part of Ramadaan because the Prophet (saws) would make 'Itikaaf during the last ten days of Ramadaan until Allaah took his soul (in death). 
1. It's observance is not legislated except in the masaajid, based on Allaah's saying: "And do not have intercourse with them (your wives) while you are making 'Itikaaf in the masaajid."  And 'Aa'ishah (raa) said: "The Sunnah for the one doing 'Itikaaf is that he should not go out (of the masjid) except for some need that he must fulfill. He should not witness a funeral, nor should he touch his wife, or have intercourse with her. And there is no 'Itikaaf except in a masjid that establishes the Jamaa'ah (congregational prayer). And the Sunnah for the one doing 'Itikaaf is that he should be fasting (on the day he makes 'Itikaaf)." 
2. The masjid should also hold the Jumu'ah prayers so that he is not forced to leave the masjid to pray the Jumu'ah prayer. This is because going out for it is an obligation, based on 'Aa'ishah's saying in one of the narrations from the previous hadeeth: "...and there is no 'Itikaaf except in a masjid that holds the Jumu'ah prayer." 
Furthermore, I came upon an authentic hadeeth that clearly specifies the "masaajid" mentioned in the previous ayah to mean the three masaajid: Masjid Al-Haraam, Masjid An-Nabawee and Masjid Al-Aqsaa. The hadeeth is: "There is no 'Itikaaf except in the three masaajid." 
According to what I came across, those among the Salaf who held this opinion were Hudhayfah Ibn Al-Yamaan, Sa'eed Ibn Al-Musayyib and 'Ataa. However, he ('Ataa) did not mention Masjid Al-Aqsaa. Others held the opinion that it was any congregating masjid (in which the Jumu'ah prayer is held), without restriction. And yet others disagreed saying it can even be done in the masjid of one's home. And there is no doubt that adhering to what complies with the hadeeth about it is what deserves to be followed. And Allaah, Glorified and Exalted, knows best.
3. It is from the Sunnah for the one making 'Itikaaf to observe fasting, as has been stated by 'Aa'ishah (raa). 
What is permissible for the person making 'Itikaaf:
1. It is permissible for him to leave the masjid in order to fulfill a need. It is also permissible for him to stick his head out of the masjid to have it washed and combed. 'Aa'ishah (raa) said: "The Messenger of Allaah (saws) entered his head in my presence while he was making 'Itikaaf in the masjid, and I was in my apartment. So I would comb his hair, [and in one narration: I would wash it, even though there was the threshold between me and him, and I would be menstruating], and he (saws) would not enter the house unless he had a [human] need, while making 'Itikaaf." 
2. It is permissible for the one making 'Itikaaf and others to make wudoo (ablution) in the masjid, based on the saying of a man who would serve the Prophet (saws): "The Prophet would make a light wudoo (ablution) in the masjid." 
3. He may set up a small tent in the back of the masjid in which to make 'Itikaaf. This is since 'Aa'ishah (raa) would set up a khibaa  (tent) for the Prophet (saws) when he would make 'Itikaaf. And this was from his (saws) command. 
And one time he made 'Itikaaf in a small tent that had a mat covering its doorway. 
The Allowance of a woman to make 'Itikaaf and visit her husband (who is making 'Itikaaf) in the masjid:
4. It is permissible for a woman to visit her husband while he is making 'Itikaaf. And he may walk with her to bid her farewell up to the door of the masjid, based on the saying of Safiyyah (raa):
"The Prophet (saws) was making 'Itikaaf in the masjid during the last ten days of Ramadaan, so I went to visit him one night, and his wives were with him. So I spoke to him for an hour, then I got up to leave and he said: 'Don't rush, I'll go out with you.' So he got up with me to escort me out." And her dwelling used to be in the home of Usaamah Ibn Zayd. Then they walked until they reached the door of the masjid, which was by the door of Umm Salamah. Then two men from the Ansaar passed by and when they saw the Prophet (saws), they rushed away. So the Prophet (saws) said: 'Slow down! This is (my wife) Safiyyah Bint Huyai' So they said: 'SubhaanAllaah, O Messenger of Allaah.' He (saws) said: 'Verily, the Devil flows through the human like the flowing of blood. And I feared lest he put some evil into your hearts.'" [Or he (saws) said: "something into your hearts."] 
In fact it is even permissible for a woman to make 'Itikaaf along with her husband in the masjid or by herself, based on the statement of 'Aa'ishah (raa): "One of the wives of Allaah's Messenger (saws) made 'Itikaaf with him while she was in the state of Istihaada (bleeding between periods) [in another narration it states she is Umm Salamah] and she would see red (blood) or yellowish traces (come out of her). And sometimes we put a tray beneath her when she offered the prayer." 
And she also said: "The Prophet (saws) would observe 'Itikaaf during the last ten days of Ramadaan, until he died. Then his wives observed 'Itikaaf after him." 
There is proof in this that it is permissible for the women to make 'Itikaaf also. And there is no doubt that this is on the condition that their guardians grant them permission to do that. And it is also on the condition that the place is free from any fitnah and intermingling with men, based on the many evidences for that and also on the Fiqh principle: "Preventing an evil takes precedence over bringing about a good."
5. Having sexual intercourse nullifies one's 'Itikaaf, based on Allaah's saying: "And do not have sexual relations with them (your wives) while you are making 'Itikaaf in the masaajid."
And Ibn 'Abbaas (raa) said: "If the person making 'Itikaaf has sexual relations then he nullifies his 'Itikaaf and must start it over again."  There is no recompense (kaffaarah) that is binding upon him due to a lack of anything being mentioned about that by the Prophet (saws) and his Companions.
 This is a portion of a hadeeth from 'Aa'ishah reported by Al-Bukharee, Muslim and Ibn Khuzaimah in their Saheeh collections. I have referenced it in Saheeh Sunan Abee Dawood (2127)
 Reported by Al-Bukharee, Muslim and Ibn Khuzaimah. The extra addition is from Al-Bukhaaree in one narration as is stated in my abridgment to it (995). And it is referenced in Saheeh Sunan Abee Dawood (2136-2137) also.
 Reported by Al-Bukhaaree and Ibn Khuzaimah in their Saheeh collections and it is referenced in the previously mentioned source (2126-2130)
 Reported by Al-Bukhaaree and Muslim and Ibn Khuzaimah (2223) and it is referenced in Al-Irwaa (966) and Saheeh Abee Dawood (2125)
 Surat-ul-Baqarah: 187. Imaam Al-Bukhaaree used this ayah as evidence for hat we have stated above. Al-Haafidh Ibn Hajr said: "The point of evidence found in this ayah is that if it were correct to do 'Itikaaf in someplace other than a masjid, the forbiddance of having sexual relations would not be specifically mentioned, because sexual intercourse is forbidden during 'Itikaaf according to the consensus of the scholars. So it is understood through the mentioning of "masaajid" that the meaning is that 'Itikaaf is not correct except in it the masaajid.
 Reported by Al-Baihaqee with an authentic chain, and Abu Dawood with a sound chain. The following narration from 'Aa'ishah RAA is also form Abu Dawood, and it is referenced in Saheeh Aee Dawood (2135) and Al-Irwaa (966)
 Al-Bayhaqee reported on Ibn 'Abbaas that he said: "Verily, the most hated of things to Allaah are the innovations. And verily from the innovations is making 'Itikaaf in the masaajid that are located in the houses."
 Reported by At-Tahaawee and Al-Isma'eelee and Al-Baihaqee with an authentic chain of narration on Hudhaifah Ibn Al-Yamaan RAA. And it is referenced in As-Saheehah (no. 2786) along with the narrations from the Sahaabah that comply with it, and all of them are authentic.
 Reported by Al-Baihaqee with an authentic chain of narration and Abu Dawood with a sound chain of narration. Imaam Ibn Al-Qayyim said in Zaad Al-Ma'aad: "It is not reported on the Prophet (saws) that he made 'Itikaaf while not fasting. Rather, 'Aa'ishah (raa) said: 'There is no 'Itikaaf except with fasting.' And furthermore, Allaah did not mention 'Itikaaf except with the fast. And the Prophet (saws) did not do it unless he was fasting. So the correct opinion is in the evidence, which the majority of the scholars adhere to: that fasting is a condition for 'Itikaaf. And this is the opinion that Shaikh-ul-Islaam Abul-'Abbaas Ibn Tamiyyah favored." And he adds to this that it is not legislated in the Religion that one who goes out to the masjid for prayer or other than that is supposed to make an intention for the amount of time he will spend in 'Itikaaf. This is what Shaikh-ul-Islaam clarified in his Ikhtiyaaraat.
 Reported by Al-Bukhaaree, Muslim, Ibn Abee Shaybah, and Ahmad, and the first addition belongs to these last two (mentioned). It is referenced in Saheeh Abee Dawood (2131-2132)
 Reported by Al-Bayhaqee with a good chain and Ahmad (5/364) in abridged form with an authentic chain of narration.
 A khibaa is one type of house of the Arabs made from fur or wool, and it is not from hair. And it is set up on two or three props. See An-Nihaayah.
 Reported by Al-Bukhaaree and Muslim from the hadeeth of 'Aa'ishah. Her doing it is stated in the report of Al-Bukhaaree and his commanding it is stated in the report of Muslim.
 It is part of a hadeeth narrated by Abu Sa'eed Al-Khudree, reported by Muslim and Ibn Khuzaimah in their Saheehs. It is referenced in Saheeh Abee Dawood (1251)
 Reported by Al-Bukhaaree and Muslim and also Abu Dawood, and the last part of it is form him (Abu Dawood). And I referenced it in Saheeh Sunan Abee Dawood (2133 & 2134).
 Reported by Al-Bukhaaree and it is referenced in Saheeh Sunan Abee Dawood (2138). The other narration is from Sa'eed Ibn Mansoor, as is I stated in Fath-ul-Baaree (4/281). However, Ad-Daarimee (1/22) says that she is Zaynab, and Allaah knows best.
 Reported by Al-Bukhaaree, Muslim and others.
 Reported by Ibn Abee Shaybah (3/92) and 'Abdur-Razaaq (4/363) with an authentic chain of narration. And what is meant by his saying: "must start it over" is that he must redo his 'Itikaaf.