Maliks Muwatta Book 18, Hadith Number 12.
Section : Fasting When Junub in the Morning during Ramadan.
Yahya related to me from Malik from Sumayy, the mawla of Abu Bakr, from Abu Bakr ibn Abd ar-Rahman that A’isha and Umm Salama, the wives of the Prophet, may Allah bless him and grant him peace, said, “The Messenger of Allah, may Allah bless him and grant him peace, used to get up in the morning junub from intercourse, not a dream, and would then fast.”