The Book of Fasting (Kitab Al-Sawm)
Muslim :: Book 6 : Hadith 2581
Abu Salama reported: I asked 'A'isha (Allah be pleased with her) about the 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 (never to break), and he did not observe fast till we said: He has given up perhaps never to fast, and I never saw him observing (voluntary fasts) more in any other month than that of Sha'ban. (lt appeared as if) he observed fast throughout the whole of Sha'ban except a few (days).