Sahih Muslim Book 6, Hadith Number 2578.

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

Abdullah b. Shaqiq reported. I asked ‘Aisha (Allah be pleased with her) about fasting of the Messenger of Allah (may peace be upon him). She said: He used to observe fast (at times) so continuously that we said: He has fasted, he has fasted. And (at times) he did not observe fast (for days) and we began to say: He has abandoned fasting, he has abandoned fasting. She said: I did not see him observing fast throughout the whole of the month since he arrived in Medina, but that of Ramadan.