Musnad Ahmad 30:2789

Muhammad ibn Ja'far narrated to us, he said: Shu'bah narrated to us, from Abu Shamir al-Dab'i, he said: I heard Abu 'Uthman al-Nahdi narrating from Abu Hurayrah, he said: "My intimate friend (the Prophet) advised me with three things: the Witr prayer before sleeping, two units of Duha prayer, and fasting three days of every month."