Musnad Abu Ya'la al-Mawsili 150:47

Muhammad ibn 'Abbad narrated to us, Sufyan narrated to us, from Sumayy, who heard Abu Bakr ibn 'Abd al-Rahman informing from 'Aishah, that the Messenger of Allah (may Allah bless him and grant him peace) would be overtaken by the morning while he was in a state of major ritual impurity (junub), and then he would fast.