Imam Al-Nawawi's Riyad-us-Saliheen
Chapter 224
Matters relating to As-Saum (Fasting)

1242. Abu Hurairah (May Allah be pleased with him) reported: The Prophet (sallallaahu ’alayhi wa sallam) said, "If any one of you forgetfully eats or drinks (while observing fasting) he should complete his Saum (fasting), for Allah has fed him and given him to drink.''
[Al-Bukhari and Muslim].

Commentary: This Hadith tells us of a kindness and convenience of Islam that during the Saum if a person inadvertently does any such thing which destroys his Saum, i.e., eating, drinking, cohabitation with wife, etc., his Saum will remain intact due to forgetfulness, provided one abandons the act as soon as he remembers that he is observing Saum . He needs not make any expiation for such a Saum.

1243. Laqit bin Sabirah (May Allah be pleased with him) reported: I requested the Messenger of Allah (sallallaahu ’alayhi wa sallam) to talk to me about ablution. He said, "Perform the Wudu' well (by washing those parts of the body, such as the face, hands and feet beyond what is required, like washing the hands up to the upper-arm instead of the elbow). Cleanse the base of your fingers and sniff water deep into the nose except when you are observing fast.''
[Abu Dawud and At-Tirmidhi].

Commentary: Ordinarily, it is necessary for Wudu' that one should pass water through the nose and rinse mouth thoroughly. But when one is observing Saum, he has to take care that water does not go down the throat through the nose as this nullifies the Saum.

1244. `Aishah (May Allah be pleased with her) reported: The Messenger of Allah (sallallaahu ’alayhi wa sallam) would wake up at Fajr time in a state of Janabah; so he would take bath before dawn and observe fasting.
[Al-Bukhari and Muslim].

1245. `Aishah, and Umm Salamah (May Allah be pleased with them) reported: The Messenger of Allah (sallallaahu ’alayhi wa sallam) would wake during Ramadan (fasting) in a state of major impurity without a wet dream, but on account of sexual intercourse and he would take a bath before dawn.
[Al-Bukhari and Muslim].

Commentary:  This Hadith sometimes and the one preceding it inform us that when the Prophet (sallallaahu ’alayhi wa sallam) would awake in the morning and was in need of bath as a result of coition with his wife, he would take Sahur in that condition and then take a bath and perform Fajr prayer, because purification is essential for Salat. This condition of Janabah (impurity) occurred without Ihtilam (wet dream) and its cause was coitus with his wife because, according to a well-known saying, Ihtilam is the result of Satan's suggestions from which Prophets are safe.