Musnad Abu Ya'la al-Mawsili 150:129

'Abd al-A'la narrated to us, Sufyan ibn 'Uyaynah narrated to us, from Ibn Abi Labid, from Abu Salamah, who said: I asked 'Aishah about the fasting of the Messenger of Allah (may Allah bless him and grant him peace). She said: "He used to fast until we would say he has fasted, and he would break his fast until we would say he has broken his fast. I never saw him fast more in any month than his fasting in Sha'ban. He used to fast all of Sha'ban; he used to fast Sha'ban, except for a little."