Musnad Ahmad 30:1265

Abu al-Nadr narrated to us, Shayban narrated to us, from Asim, from al-Aswad ibn Hilal, from Abu Hurayrah, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, commanded me with three things: sleeping upon a Witr, performing Ghusl on the day of Jumu'ah, and fasting three days of every month."