Musnad Abu Ya'la al-Mawsili 209:84

Abu Khaythama narrated to us, Jarir narrated to us, from Yahya, from 'Arak bin Malik, from 'Abd al-Malik bin Abi Bakr, from his father, from Umm Salamah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) would wake up in the morning in a state of sexual impurity (junub) from his wives, without having had a wet dream, and then he would remain fasting.