Musnad Ahmad 30:394

'Abd al-Wahid al-Haddad narrated to us, from Khalaf ibn Mihran, who said: I heard 'Abd al-Rahman ibn al-Asamm say: Abu Hurayrah said: My intimate friend (the Prophet) advised me with three things: fasting three days of every month, the Duha prayer, and not to sleep unless I have performed Witr.