Musnad Ahmad 1153:53

'Abd al-A'la narrated to us, from Ma'mar, from al-Zuhri, from Abu Bakr ibn 'Abd al-Rahman ibn al-Harith ibn Hisham, who said: I and my father entered upon 'Aishah and Umm Salamah, and they both said: "The Prophet, peace and blessings of Allah be upon him, would wake up in a state of janabah (major ritual impurity), then he would fast."