Volume 3, Book 31, Number 202:
Narrated Abu Huraira (radiallaahu `anhu):
My friend (the Prophet (sallallaahu `alayhi wasallam)) advised me to observe three things:
(1) to fast three days a month;
(2) to pray two Rakat of Duha prayer (fore-noon prayer); and
(3) to pray Witr before sleeping.
it is mustahabb to fast three days each month, and it is best to fast the ayaam al-beed, which are the 13th, 14th and 15th of the month.
It was narrated that ‘Abd-Allaah ibn ‘Amr ibn al-‘Aas (may Allaah be pleased with him) said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said to me:
“It is sufficient for you to fast three days every month, because for every good deed you will have (the reward of) ten like it, so that will be like fasting for a lifetime.”
Narrated by al-Bukhaari (1874); Muslim (1159).
Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allaah have mercy on him) was asked:
It was narrated in the hadeeth that the Prophet (peace and blessings of Allaah be upon him) advised Abu Hurayrah (may Allaah be pleased with him) to fast three days of each month. When should these fasts be observed, and should they be one after the other?
It is permissible to fast these days one after the other or separately, and they may be at the beginning of the month, or in the middle, or at the end. The matter is broad in scope, praise be to Allaah, because the Messenger of Allaah (peace and blessings of Allaah be upon him) did not state any specific days. ‘Aa’ishah (may Allaah be pleased with her) was asked: Did the Messenger of Allaah (peace and blessings of Allaah be upon him) fast three days of each month? She said: Yes. It was asked: In which part of the month did he fast? She said: He would not bother in which part of the month he would fast. Narrated by Muslim, (1160). But the 13th, 14th and 15th are better, because these are al-ayyaam al-beed.
Majmoo’ Fataawa al-Shaykh Ibn ‘Uthaymeen, 10/ question no. 376.