Shama-il Tirmidhi Chapter 042, Hadith Number 006 (286).

'Abdullah bin Mas'ud Radiyallahu 'Anhu reports: "Rasulullah Sallallahu 'Alayhi Wasallam fasted for three days at the beginning of every month, and he very seldom did not fast on Friday".

Commentary
One has been persuaded in many ahaadith to observe saum for three days in every month. The thawaab (reward) for every good deed is tenfold, therefore the fasting of three days will add up to the thawaab of thirty days. In this manner it will be as if one has fasted for one's whole life. Many ahaadith have been mentioned regarding the fixing of the three days. There are no differences in this. At times on every Monday and Thursday. Sometimes on the thirteenth, fourteenth and fifteenth. In the same way he fasted at other times too. It is mentioned in an ensuing hadith by Sayyiditina 'Aayeshah Radiyallahu 'Anha that Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam did not fix special days for fasting. Sayyidina Ibn 'Abbaas Radiyallahu 'Anhu says: 'Rasulullah Sallallahu 'Alayhi Wasallam did not omit the fasting of the ayyaami beed (13th, 14th and 15th day of the Islaamic lunar month) at home or while on his travels'. The second point in the above hadith is the fasting on Fridays. From this hadith it is found that the fast of Fridays were given importance, but in another hadith it has been prohibited to fast on friday only. For this reason some of the 'ulama, due to the above hadith are of the opinion that it is mustahab to fast on friday and some 'ulama due to the hadith wherein it has been prohibited, are of the opinion that it is makruh to fast on friday only. The Hanafis hold both opinions.