Sahih Muslim Book 6, Hadith Number 2581.

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

Abu Salama reported: I asked ‘Aisha (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.(It appeared as if) he observed fast throughout the whole of Sha’ban except a few (days).