Musnad Ahmad 30:3143

Yunus narrated to us, Al-Khazraj narrated to us, from Abu Ayyub, from Abu Hurairah, who said: My intimate friend, Abu al-Qasim (may Allah bless him and grant him peace), advised me with three things that I will not abandon: Performing ghusl (full ritual bath) on the day of Jumu'ah, fasting three days of every month, and performing the Witr prayer before sleeping.