Musnad Ahmad 30:3428

Yazid narrated to us, Al-Awwam informed us, Sulayman ibn Abi Sulayman narrated to us, that he heard Abu Hurayrah saying: "My intimate friend (Khalil), may Allah bless him and grant him peace, enjoined upon me three things, and I will not abandon them in travel or at home: that I do not sleep except upon a Witr (prayer), that I fast three days of every month, and that I do not leave the two Rak'ahs of Ad-Duha, for it is the prayer of the oft-returning (to Allah)."