Musnad Ahmad 1301:4

Abu al-Mughirah narrated to us, he said: Safwan narrated to us, he said: Some of the elders narrated to me, from Abu Idris al-Khawlani, from Jubayr ibn Nufayr, from Abu al-Darda’, who said: My intimate friend, Abu al-Qasim (may Allah bless him and grant him peace), enjoined upon me three things that I will not abandon for anything: He enjoined upon me to fast three days of every month, and not to sleep except after performing Witr, and the Duha prayer while at home and while traveling.