Musnad Ahmad 1153:2235

Yunus narrated to us, he said: Hammad—meaning Ibn Salamah—narrated to us, from ‘Asim ibn Bahdalah, from Abu Salih, from ‘Aishah, that she said: "The Messenger of Allah (peace and blessings of Allah be upon him) would go out for the dawn prayer while his head was dripping from sexual intercourse, not from a wet dream."