Musnad Ahmad 1156:138

Yahya and Waki’ narrated to us, they said: Usamah ibn Zayd narrated to us, he said: Sulayman ibn Yasar narrated to us, that he heard Umm Salamah saying—Waki’ said in his narration: He said: I heard Sulayman ibn Yasar, from Umm Salamah, she said: "The Messenger of Allah (peace and blessings of Allah be upon him) would touch his wives at night, then he would wake up in a state of janabah (major ritual impurity) without having had a wet dream, so he would perform ghusl (ritual bath) and fast."