Musnad Ahmad 1153:740

Ishaq ibn Isa narrated to us, Malik narrated to us, from Abu al-Nadr, from Abu Salamah ibn Abd al-Rahman, from Aisha, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) would fast until we would say he does not break his fast, and he would break his fast until we would say he does not fast. He never completed the fasting of an entire month except for Ramadan, and I never saw him fast more in any month than he did in Sha'ban."