Musnad Ahmad 1156:11

'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 'A'ishah and Umm Salamah. They both said: "The Prophet, may Allah bless him and grant him peace, would wake up in a state of janabah (major ritual impurity), and then he would fast."