A Sahaabee (Companion) is anyone that met the Prophet (sallAllaahu ‘alayhi wa sallam) while believing in him, and dying upon that condition.
The Companions (Sahaabah) of the Prophet (sallAllaahu ‘alayhi wa sallam) are the best from all those that accompanied the prophets, due to the statement of Allaah’s Messenger (sallAllaahu ‘alayhi wa sallam): “The best of people are my Companions.”  Al-Bukhaaree and others have reported this hadeeth.
The best among the Sahaabah are the Muhaajiroon (those who performed the migration from Makkah to Madeenah), due to their combining between migrating and giving support (in Allaah’s Cause). Then after them, are the Ansaar.
The best among the Muhaajiroon are the four righteous Khaleefahs: Abu Bakr, ‘Umar, ‘Uthmaan and ‘Alee.
Abu Bakr – He is “As-Siddeeq” ‘Abdullaah bin ‘Uthmaan bin ‘Aamir from the Tribe of Taym bin Marrah bin Ka’ab. He was the first male to believe in the Messenger of Allaah (sallAllaahu ‘alayhi wa sallam). He was his companion in the migration (Hijrah) and his deputy during times of prayer and Hajj. He was his Khaleefah (designated successor) for his nation. Five of those who were given the glad tidings of Paradise accepted Islaam through his hands.
They are ‘Uthmaan, Az-Zubair, Talha, ‘Abd-ur-Rahmaan bin ‘Awf and Sa’ad bin Abee Waqqaas. He died in Jumaadal-Al-Akhirah, in the 13th year of Hijrah, when he was 63 years old. The above five along with Abu Bakr, ‘Alee bin Abee Taalib and Zayd bin Haarithah form the eight that were the first of people to accept Islaam. Ibn Ishaaq stated this and what he meant by it, was that they were the first from amongst the males, after (the advent of) the Message.
‘Umar – He is Abu Hafs “Al-Faarooq” ‘Umar Ibn Al-Khattaab from the tribe of ‘Adiyy bin Ka’ab bin Lu’aa. He accepted Islaam in the sixth year after the Prophet (sallAllaahu ‘alayhi wa sallam) was sent, after nearly forty men and eleven women (had accepted Islaam). The Muslims were overjoyed by that occasion and Islaam spread in Makkah afterward. Abu Bakr entrusted him with the leadership of the Muslim nation, and so he rose to the hardship of the Khilaafah in the best manner. This was until he was killed and died as a martyr in Dhul-Hijjah of the year 23H, at the age of 63.
‘Uthmaan – He is Abu ‘Abdillaah “Dhun-Noorain” ‘Uthmaan bin ‘Affaan from the tribe of Umayyah bin ‘Abd-ish-Shams bin ‘Abd Manaaf. He accepted Islaam after the Prophet’s entrance into Daar-ul-Arqam. He was a rich and generous man. He was given the Khilaafah after ‘Umar Ibn Al-Khattaab (radyAllaahu ‘anhu), due to the agreement of the members of the governing body. He remained Khaleefah until he was killed and martyred in Dhul-Hijjah in the year 35H. He was 90 years old at the time of his death, according to one of several opinions.
‘Alee – He is Abul-Hasan ‘Alee bin Abee Taalib. The name of Abu Taalib was ‘Abd Manaaf bin ‘Abd-il-Muttallib. He was the first from the youth to accept Islaam. The Messenger of Allaah (sallAllaahu ‘alayhi wa sallam) gave him the banner on the Day of Khaibar and Allaah granted victory at his hands. He was given the Khilaafah after the murder of ‘Uthmaan. Thus, he became the Khaleefah, governing until he was killed himself, and martyred in the month of Ramadaan in the 40H, at the age of 63.
The best of these four is Abu Bakr, then ‘Umar, then ‘Uthmaan, then ‘Alee. This is based on the report of Ibn ‘Umar (radyAllaahu ‘anhu): “We used to make preference between the people during the time of the Prophet (sallAllaahu ‘alayhi wa sallam). So we preferred Abu Bakr, then ‘Umar Ibn Al-Khattaab, then ‘Uthmaan bin ‘Affaan.” This was reported by Al-Bukhaaree. It is also based on (the hadeeth) in Abu Dawood: “We used to say, while the Messenger of Allaah (sallAllaahu ‘alayhi wa sallam) was alive: The best of the Prophet’s ummah after him, is Abu Bakr, then ‘Umar, then ‘Uthmaan.” At-Tabaraanee added to this in his report: “The Prophet would hear that and he would not oppose it.” I did not find the wording that the author mentions, in which there is the addition of ‘Alee bin Abee Taalib. 
The most deserving from them of the Khilaafah, after the Prophet, was Abu Bakr (radyAllaahu ‘anhu). This is since he was the best of them and foremost to them in accepting Islaam. Also, the Prophet (sallAllaahu ‘alayhi wa sallam) preferred him to lead the prayer. Furthermore, the Sahaabah agreed upon his superiority and upon giving him the oath of allegiance (bay’ah). And it is not befitting for Allaah to unite them in agreement upon an error. Then ‘Umar, since he was the best of the Sahaabah after Abu Bakr and because Abu Bakr entrusted the Khilaafah to him. Then ‘Uthmaan, due to his merits and due to his election by the Majlis Ash-Shooraa (governing body). They are the ones mentioned in this verse from a poem:
“’Alee, ‘Uthmaan, Sa’ad and Talha.
Zubair and Dhu ‘Awf — members of the governing body.”
Then comes ‘Alee, due to his merits and the consensus of the Muslims of his time upon it. These four are the rightly guided Khaleefahs, whom the Prophet (sallAllaahu ‘alayhi wa sallam) said concerning them: “Stick to my Sunnah and the Sunnah of the rightly-guided Khaleefahs after me. Bite onto it (the Sunnah) with your molar teeth.” And he (sallAllaahu ‘alayhi wa sallam) said: “The Khilaafah after me will endure for thirty years.” Ahmad, Abu Dawood and At-Tirmidhee reported this hadeeth and Al-Albaanee said that its chain of narration was hasan. Thus, the last part of it, was the Khilaafah of ‘Alee. This is as the author has stated and it is as if he places the Khilaafah of Al-Hasan subordinate to his father’s or just doesn’t acknowledge it (i.e. Al-Hasan’s Khilaafah) at all, due to his yielding it.
The Khilaafah of Abu Bakr (radyAllaahu ‘anhu) endured for two years, three months and nine nights, from the 13th of Rabee’-ul-Awwal 11H to the 22nd of Jumaadal-Akhirah 13H.
The Khilaafah of ‘Umar (radyAllaahu ‘anhu) lasted ten years, six months and three days, from the 23rd of Jumaadal-Akhirah 13H to the 26th of Dhul-Hijjah 23H.
The Khilaafah of ‘Uthmaan (radyAllaahu ‘anhu) lasted twelve years minus 12 days, from the month of Muharram 24H to the 18th of Dhul-Hijjah 35H.
The Khilaafah of ‘Alee (radyAllaahu ‘anhu) lasted four years and nine months from the 19th of Dhul-Hijjah 35H to the 19th of Ramadaan 40H.
Combined, the Khilaafah of these four (Companions) lasted twenty-nine years, six months and four days.
Thereafter, Al-Hasan bin ‘Alee (radyAllaahu ‘anhu) was given the oath of allegiance on the day that his father, ‘Alee died. Then, in the month of Rabee’-ul-Awwal 41H, the command was given to Mu’awiyah (radyAllaahu ‘anhu) and due to that, the signs which the Prophet (sallAllaahu ‘alayhi wa sallam) spoke of in his statement: “The Khilaafah after me will endure for thirty years” and his statement: “Indeed, this son of mine is a Sayyid (noble) and perhaps it will be that Allaah will bring peace between two large parties (i.e. that of Al-Hasan and Mu’awiyah) through him” came to pass.
Al-Hasan was the grandson of the Messenger of Allaah (sallAllaahu ‘alayhi wa sallam) and the fruit of his offspring. He was the Ameer-ul-Mu’mineen(Commander of the Believers), son of the Ameer-ul-Mu’mineen ‘Alee bin Abee Taalib. He was born on the 15th of Ramadaan in the third year after Hijrah. He died while in Madeenah and was buried in the graveyard Al-Baqee’ in Rabee-ul-Awwal 50H.
Al-Husayn was the grandson of the Messenger of Allaah (sallAllaahu ‘alayhi wa sallam) and the fruit of his offspring. He was the son of ‘Alee bin Abee Taalib. He was born in Sha’baan in the fourth year after Hijrah and was killed at Karbala on the tenth of Muharram 61H.
 Saheeh Al-Bukhaaree: Book of the Virtues of the Companions of the Prophet (no. 3651) and Saheeh Muslim: Book of the Virtues of the Companions (no. 2533) from the hadeeth of Ibn Mas’ood (radyAllaahu ‘anhu). It is also reported on the authority of ‘Imraan bin Husayn (radyAllaahu ‘anhu) in Al-Bukhaaree (no. 2561, 3650, 6438 & 6695) and Muslim (no. 2535). Abu Hurairah (radyAllaahu ‘anhu) also reported it in Muslim (no. 534) as well as others. Furthermore, this is a mutawaatir hadeeth. Al-Haafidh Ibn Hajr stated the correctness of it being mutawaatir in the introduction to his Al-Isaabah. Saheeh Al-Bukhaaree: Book of the Virtues of the Companions (no. 3655). And in another wording found in Al-Bukhaaree (no. 3697), it states: “During the lifetime of the Prophet, we used to not put anyone equal to Abu Bakr, then (the same was with) ‘Umar, then (the same was with) ‘Uthmaan. Then we left the (remaining) Companions of the Prophet alone, not distinguishing between any of them.”
 Isnaaduhu Saheeh: This is an authentic hadeeth reported by Abu Dawood (4628), At-Tirmidhee (3707) and Ibn Abee ‘Aasim in As-Sunnah (1190). Its chain of narration (isnaad) is authentic as has been stated by Al-Albaanee in his checking to As-Sunnah (2/567). As for the addition that Shaikh Ibn Al-‘Uthaimeen has mentioned, which is found in At-Tabaraanee, which is: “The Prophet would hear that and he would not oppose it”, then it is an authentic addition, which is established in many paths of narration found in Ibn Abee ‘Aasim (1194, 1195, 1196 & 1197), Ahmad (2/14) and others with authentic chains. Refer to the checking of Ibn Abee ‘Aasim’s As-Sunnah (2/568-569) as well as Fath-ul-Baaree (7/16-17)
 Saheeh Al-Bukhaaree: Book of Peace Making (no. 2704) from the hadeeth of Abu Bakrah (radyAllaahu ‘anhu).
Imaam Muhammad Ibn Saalih Al-‘Uthaimeen
Source: His Book Sharh Lum’at-ul-‘Itiqaad (pg. 140-143)
Translator: Isma’eel Alarcon