Musnad Ahmad 30:2788

Muhammad ibn Ja'far narrated to us, he said: Shu'bah and Abu Dawud narrated to us, they said: Shu'bah narrated to us, from 'Abbas—meaning al-Jurayri—he said: I heard Abu 'Uthman narrating from Abu Hurayrah, he said: "My intimate friend (Khalil), may Allah bless him and grant him peace, enjoined upon me three things: the Witr prayer before sleeping, the two rak'ahs of Duha, and fasting three days of every month."