Musnad Ahmad 1156:192

Rawh narrated to us, Ibn Jurayj narrated to us, he said: Ibn Shihab narrated to me, from Abu Bakr ibn Abd al-Rahman, from his father, from Umm Salamah and Aisha, the wives of the Prophet, may God bless him and grant him peace: That the dawn would overtake him while he was in a state of sexual impurity (junub) from his family, then he would perform ghusl (ritual bath) and fast.