Sahih Al-Bukhari – Volume 3 – Book 30 – P.123
(58) CHAPTER. Saum (fasting) on alternate days.
1978. Narrated Mujãhid from ‘Abdulläh bin ‘Amr: The Prophet, peace be upon you, said (to ‘Abdullah), “Observe Saum (fast) three days a month.” ‘Abdullah said, (to the Prophet, peace be upon him ) “I am able to observe Saum (fast) more than that.” They kept on arguing on this matter till the Prophet, peace be upon him, said, “Observe Saum (fast) on alternate days, and recite the whole Qur’an once a month.” ‘Abdullah said, “I can recite more (in a month) ,” and the argument went on till the Prophet, peace be upon him, said, “Recite the whole Qur’an once in three days.” (i.e., you must not recite the whole Qur’an in less than three days).
(59) CHAPTER. The Saum (fasting) of Dãwud (David),peace be upon him
1979. Narrated ‘Abdullah bin ‘Amr bin Al-‘As : The Prophet, peace be upon him, said to me, “You observe Saum (fast) daily all the year and offer Salat (prayer) (every night) all night?” I replied in the affirmative. The Prophet, peace be upon him, said, “If you keep on doing this, your eyes will become weak and your body will get tired. He who observes Saum (fast) all the year is as he who did not observe Saum (fast) at all. Observing Saum (fast) of three days (a month) will be equal to observing Saum (fast) of the whole year.” I replied, “I have the strength for more than this.” The Prophet, peace be upon him, said, “Then observe Saum (fast) like the fasting of Dawud (David), peace be upon him, who used to observe Saum (fast) on alternate days and would never flee from the battle field on meeting the enemy.”
Sahih Al-Bukhari – Volume 3 – Book 30 – P.124
(60) CHAPTER. To observe Saum (fast) the three days (preceding) the full moon night, i.e., 13th, 14th and the 15th of the lunar months.
1981. Narrated Abu Hurairah, may Allah be pleased with him: My friend (the Prophet, peace be upon him) advised me to observe three things:
(1) to observe Saum (fast) three days every (lunar) month;
(2) to perform a two Rak’a Duha prayer and
(3) to perform the Witr prayer before sleeping.
Sahih Al-Bukhari – Volume 3 – Book 30 – P.125
(61) CHAPTER. Whoever visited some people and did not break his (optional) Saum (fast) with them.
1982. Narrated Anas, may Allah be pleased with him: The Prophet, peace be upon him, paid a visit to Umm Sulaim and she placed before him dates and ghee. The Prophet, Peace be upon him said, “Replace the ghee and dates in their respective containers for I am observing Saum (fast).” Then he stood somewhere in her house and offered an optional Salat (prayer) and then he invoked Allah to bless Umm Sulaim and her family. Then Umm Sulaim said, “O Allah’s Messenger! I have a special request (today).” He, peace be upon him, said, “What is it?” She replied, “(Please invoke for) your servant Anas.” So, Allah’s Messenger, peace be upon him, did not leave anything good in this world or in the Hereafter which he did not invoke (Allah to bestow) on me and said, “O Allah! Give him (i.e., Anas) property and children and bless him.” Thus I am one of the richest among the Ansar and my daughter Umaina told me that when Al- Hajjaj came to Basrah, more than 120 of my offspring had been buried.
Sahih Al-Bukhari – Volume 3 – Book 30 – P.126
(62) CHAPTER. Fasting the last days of the month.
1983. Narrated ‘Imrãn bin Husain,may Allah be pleased with them, that the Prophet, peace be upon him, asked him (‘Imrãn), or asked a man and ‘Imrãn was listening, “O Abu so-and so! Have you observed Saum (fasts) in the last days of this month?” (The narrator thought that he said, “the month of Ramadan”). The man replied, “No. O Allah’s Messenger!” The Prophet, peace be upon him, said to him, “When you finish your Saum (fasts) (of Ramadan) observe Saum (fasts) for two days (in Shawwãl) .” Through another series of narrators ‘Imrãn said, “The Prophet said, “[Have you observed Saum (fasts)] in the last days of Sha’bãn?’ “
(63) CHAPTER. Observing Saum (fast) on Friday. If someone gets up in the morning of Friday and is observing the Saum (fasts) he should break it [if he did not observe Saum the day before or does not intend to observe Saum (fasts) after it].
1984. Narrated Muhammad bin ‘Abbad : I asked Jãbir, may Allah be pleased with him, “Did the Prophet, peace be upon him, forbid observing Saum (fasts) on Fridays?” He replied, “Yes.” [Through other narrators it is added, “If he intends to observe Saum (fasts) only that day.”]
Sahih Al-Bukhari – Volume 3 – Book 30 – P.127
1985. Narrated Abu Hurairah, may Allah be pleased with him, I heard the Prophet, peace be upon him, saying, “None of you should observe Saum (fast) on Friday unless he observes Saum (fast) a day before or after it.”
1986. Narrated Juwairiya bint Al-Harith, may Allah be pleased with her: The Prophet, peace be upon him, visited her (Juwairiya) on a Friday and she was observing Saum (fast). He, peace be upon him,asked her, “Did you observe Saum (fast) yesterday?” She said, “No.” He, peace be upon him, said, “Do you intend to observe Saum (fast) tomorrow?” She said “No.” He, peace be upon him, said, “Then break your Saum (fast) .”
Through another series of narrators, Abu Ayyub is reported to have said, “He, peace be upon him, ordered her and she broke her Saum (fast).”