Imam Al-Nawawi's Riyad-us-Saliheen
Chapter 230
The Desirability of Observing Three Days of Fasting in Every Month

1258. Abu Hurairah (May Allah be pleased with him) reported: My friend (the Messenger of Allah) (sallallaahu ’alayhi wa sallam) directed me to observe fast for three days in every month, to perform two Rak`ah (optional) Duha prayer at forenoon and to perform the Witr prayer before going to bed.
[Al-Bukhari and Muslim].

1259. Abud-Darda (May Allah be pleased with him) reported: My friend (the Messenger of Allah) (sallallaahu ’alayhi wa sallam) directed me to observe Saum (fasting) for three days in every month, to perform two Rak`ah (optional) Duha prayer at forenoon, and to perform the Witr prayer before going to bed.
[Al-Bukhari and Muslim].

1260. `Abdullah bin `Amr bin Al-`As (May Allah be pleased with them) reported: The Messenger of Allah (sallallaahu ’alayhi wa sallam) said, "Observing Saum (fasting) on three days of every month is equivalent to a full month's fasting.''
[Al-Bukhari and Muslim].

1261. Mu`adhah Al-`Adawiyah (May Allah be pleased with her) reported: I asked `Aishah (May Allah be pleased with her). Did the Messenger of Allah (sallallaahu ’alayhi wa sallam) use to observe three days of Saum (fasting) in every month?'' She replied, "Yes.'' I asked, "On which days in the month did he observed fast?'' She replied that he did not mind on which days of the month he observed fast.
[Muslim].

Commentary:  This Hadith makes it clear that one can observe Saum on any three days of the month, and it is not necessary to fix these days. However, it is more meritorious to observe Saum on 13th, 14th and 15th of each lunar month because the Prophet (sallallaahu ’alayhi wa sallam) has so ordained it. He himself also used to take special care of these dates and observe Saum on them, as is evident from the forthcoming Ahadith.

1262. Abu Dharr (May Allah be pleased with him) reported: The Messenger of Allah (sallallaahu ’alayhi wa sallam) said, "If you want to observe Saum on three days in a month, then fast on the thirteenth, fourteenth and fifteenth of the (lunar) month.''
[At-Tirmidhi].

1263. Qatadah bin Milhan (May Allah be pleased with him) reported: The Messenger of Allah (sallallaahu ’alayhi wa sallam) directed us to observe Saum (fasting) on the thirteenth, fourteenth and fifteenth of the (lunar) month.
[Abu Dawud].

1264. Ibn `Abbas (May Allah be pleased with them) reported: It was the practice of the Messenger of Allah (sallallaahu ’alayhi wa sallam) to not to omit fast on `bright days' whether he was staying at a place or on a journey.
[An-Nasa'i].

Commentary: "Beed''  is the plural of "Abyad'' which means white. Three days of 13, 14 and 15 of lunar month are called "Al-Ayyam-ul-Beed'' for the reason that their nights are bright because of the moonlight. The moon begins to diminish gradually after these days. In any case, it is evident from the Ahadith mentioned here that observance of Saum on these days is more meritorious but one can do so with benefit on other days also.