Musnad Ahmad 1153:1946

Suwayd ibn ‘Amr narrated to us, he said: Aban narrated to us, Yahya narrated to us, from Abu Salamah, from ‘Aishah, she said: "The most of the fasting of the Messenger of Allah, may Allah bless him and grant him peace, of the months of the year was Sha‘ban, for he used to fast all of it."