Musnad Ahmad 30:1238

Abu al-Nadr narrated to us, al-Mubarak narrated to us, from al-Hasan, from Abu Hurayrah, 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: fasting three days of every month, not sleeping unless I have performed Witr, and performing Ghusl on the day of Jumu'ah.