Musannaf Ibn Abi Shayba 4:711

Shababa ibn Sawwar narrated to us, from Layth ibn Sa'd, from Ibn Shihab, from Abu Bakr ibn Abd al-Rahman ibn al-Harith ibn Hisham, who said: Aisha and Umm Salama informed me that the Prophet, peace and blessings of Allah be upon him, would be overtaken by dawn while he was in a state of janaba (major ritual impurity) from his wives, then he would perform ghusl (ritual bath) and fast.