Musnad Abu Ya'la al-Mawsili 156:3

Shayban narrated to us, Jarir ibn Hazim narrated to us, al-Hasan narrated to us, from Abu Hurayrah, who said: My intimate friend (Khalil), may Allah bless him and grant him peace, enjoined upon me three things that I shall never abandon: the Witr prayer before sleep, fasting three days of every month, and performing Ghusl on Friday.