Musnad Ahmad 30:2981

Yahya narrated to us, from Imran Abu Bakr, who said: Al-Hasan narrated to us, from Abu Hurairah, who said: "My intimate friend, the Messenger of Allah (may Allah bless him and grant him peace), enjoined upon me three things: the Witr prayer before sleep, fasting three days of every month, and performing Ghusl on Friday."