Musnad Ahmad 1153:2173

Hammad narrated to us, Aflah narrated to us, from al-Qasim, from Aisha, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) would wake up in the morning while he was in a state of major ritual impurity (junub), then he would perform ghusl (ritual bath), and he would fast his day."