Musnad Ahmad 30:2094

Nuh ibn Maymun narrated to us, he said: Abdullah ibn al-Mubarak informed us, from Sufyan, from Tariq ibn Abd al-Rahman, from Zadhan, from Abu Hurayrah, who said: My intimate friend (the Prophet) advised me with three things: the Witr prayer before sleeping, fasting three days of every month, and the two units of prayer of Duha (the forenoon prayer).