Musnad Abu Ya'la al-Mawsili 209:121

Abu Khaythama narrated to us, Yahya bin Sa'id narrated to us, Shu'bah narrated to us, from Qatadah, from Sa'id bin al-Musayyib, from 'Amir, the brother of Umm Salamah, from Umm Salamah: That the Messenger of Allah (peace and blessings of Allah be upon him) would wake up in a state of sexual impurity (junub), then he would fast.