Musnad Ahmad 30:1977

Yunus narrated to us, Abd al-Aziz—meaning Ibn al-Mukhtar al-Ansari—narrated to us, from Abdullah—meaning Ibn Fayruz al-Danaj—who said: Abu Rafi‘ al-Sa’igh narrated to us, who said: Abu Hurayrah said: "Three things I memorized from my intimate friend, Abu al-Qasim (may Allah bless him and grant him peace): the Witr prayer before sleep, fasting three days of every month, and the two rak'ahs of Duha."