Musnad Ahmad 30:3352

Ali ibn 'Asim narrated to us, Layth ibn Abi Sulaym narrated to us, from Mujahid, from Abu Hurayrah, who said: My intimate friend (the Prophet), may Allah bless him and grant him peace, advised me with three things: "That I do not sleep except upon a Witr (prayer), the fasting of three days of every month, and the two Rak'ahs of Ad-Duha (the forenoon prayer)."