Musnad Ahmad 1153:1300

Ibn Numayr narrated to us, he said: Muhammad—meaning Ibn ‘Amr—narrated to us, he said: Abu Salamah narrated to us, from ‘Aishah, she said: I said: "O Mother, how was the fasting of the Messenger of Allah, may Allah bless him and grant him peace?" She said: "He 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.' And I did not see him fast in any month more than his fasting in Sha‘ban; he would fast Sha‘ban, except for a little, or rather, he would fast all of it."