Musnad Abu Ya'la al-Mawsili 159:121

Muhammad ibn al-Khattab narrated to us, Mu’ammal narrated to us, Hammad ibn Salamah narrated to us, Qays ibn Sa’d narrated to us, from ‘Ata’, from Abu Hurayrah, who said: My intimate friend (Khalil), may Allah bless him and grant him peace, enjoined upon me three things: the Witr prayer before sleeping, fasting three days of every month, and the two rak'ahs of Duha.