There follow a number of ahaadeeth which encourage fasting onMondaysand Thursdays:
1 – it was narrated from Abu Qataadah al-Ansaari that the Messenger of Allaah (peace and blessings of Allaah be upon him) was asked about fasting on Mondays. He said: “On that day I was born, andonit the Revelation came to me.” Narrated by Muslim, 1162.
2 – it was narrated that ‘Aa’ishah (may Allaah be pleased with her) said: The Prophet (peace and blessings of Allaah be upon him) was keen tofastonMondaysand Thursdays. Narrated by al-Tirmidhi, 745; al-Nasaa’i, 2361; Ibn Maajah, 1739; classed as saheeh by al-Albaani in Saheeh al-Targheeb, 1044.
3 – it was narrated from Abu Hurayrah (may Allaah be pleased with him) that the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Deeds are shown (to Allaah) onMondaysand Thursdays, and I like my deeds to be shown when I am fasting.” Narrated by al-Tirmidhi, 747; classed as saheeh by al-Albaani in Saheeh al-Targheeb, 1041.
The following ahaadeeth encourage fasting onthreedaysofeach month:
1 – it was narrated that Abu Hurayrah (may Allaah be pleased with him) said: “My close friend [the Prophet (peace and blessings of Allaah be upon him)] advised me to do three things which I will not give up until I die: fasting threedaysofeach month, praying Duha, and sleeping after Witr. Narrated by al-Bukhaari, 1124; Muslim, 721.
2 – it was narrated from Mu’aadhah al-‘Adawiyyah that she asked ‘Aa’ishah, the wife of the Prophet (peace and blessings of Allaah be upon him), “Did the Messenger of Allaah (peace and blessings of Allaah be upon him) fastthreedaysof every month?” She said, “Yes.” She said to her: “Which daysof the month did he fast?” She said, “He did not mind which daysof the month he would fast.” Narrated by Muslim, 1160.
3 – it was narrated from Jareer ibn ‘Abd-Allaah (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said: “Fasting threedaysofeach month Is fasting for a lifetime, and ayaam al-beed are the thirteenth, fourteenth and fifteenth.” Narrated by al-Nasaa’i, 2420; classed as saheeh by al-Albaani in Saheeh al-Targheeb, 1040.
4 – it was narrated that Abu Dharr (may Allaah be pleased with him) said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said to me: “If you fast any part of the month, then fast the thirteenth, fourteenth and fifteenth.” Narrated by al-Tirmidhi, 761; al-Nasaa’i, 2424; classed as saheeh by al-Albaani in Saheeh al-Targheeb, 1038.
There Is broad scope in the command tofastthree days, as it says in the hadeeth of ‘Aa’ishah (may Allaah be pleased with her), but the best daysof the month for fasting are the thirteenth, fourteenth and fifteenth, as it says in the other saheeh ahaadeeth.
Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allaah have mercy on him) was asked: When fasting threedaysof every month, Isit essential that it be only on the ayyaam al-beed? orIsit permissible tofast any threedaysof the month?
He replied:
It Is permissible tofast at the beginning of the month, in the middle or at the end, on consecutive daysor separately. But itIsbetter if itIs done on the three ayyaam al-beed, which are the thirteenth, fourteenth and fifteenth. ‘Aa’ishah (may Allaah be pleased with her) said: The Prophet (peace and blessings of Allaah be upon him) used tofastonthreedaysof every month, and he did not mind whether he fasted at the beginning or at the end of the month. End quote.
Majmoo’ Fataawa al-Shaykh Ibn ‘Uthaymeen, 20/question no. 376
And Allaah knows best.