Musnad Ahmad 30:3320

Mu'tamir narrated to us, from Layth, from Mujahid and Shahr, from Abu Hurayrah, who said: My intimate friend [the Prophet] advised me with three things: "That I do not sleep unless I have performed Witr, that I fast three days of every month, and that I do not abandon the two rak'ahs of Duha." 'Abdullah said: I found these two hadiths in my father's book in his own handwriting.