Musnad Ahmad 1153:2035

Rawh narrated to us, he said: Malik narrated to us, from Abu al-Nadr, the freed slave of Umar ibn Ubayd Allah, from Abu Salamah, from Aisha, that she said: "The Messenger of Allah (peace and blessings of Allah be upon him) used to 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. And I never saw the Messenger of Allah (peace and blessings of Allah be upon him) complete the fast of an entire month except for Ramadan, and I never saw him in any month fast more than he did in Sha'ban."