Sahih Muslim Book 6, Hadith Number 2584.

Chapter : The fasts observed by the Apostle of Allah (may peace be upon him) besides Ramadhan.

This Hadith has been narrated on the authority of Abu Bishr with the same chain of transmitters (with a slight variation of words and these are), that he (the narrator) said: “During any month continuously since he came to Medina.”