Musnad Ahmad 30:3681

'Abdullah ibn Yazid narrated to us, Haywah narrated to us, Abu 'Aqil, Zuhrah ibn Ma'bad, narrated to me from his father, Ma'bad ibn 'Abdullah ibn Hisham, that he heard Abu Hurayrah saying: My intimate friend (the Prophet), may Allah bless him and grant him peace, enjoined upon me three things that I will not abandon until I die: He enjoined upon me the two rak'ahs of Duha (forenoon prayer), fasting three days of every month, and that I do not sleep unless I have performed Witr.