Musnad Ahmad 1302:39

Al-Hakam ibn Nafi‘ Abu al-Yaman narrated to us, Safwan ibn ‘Amr narrated to us, from Abu Idris al-Khawlani, from Jubayr ibn Nufayr, from Abu al-Darda’, who said: "My beloved (the Prophet), may Allah bless him and grant him peace, advised me with three things that I will not abandon for anything: He advised me to fast three days of every month, not to sleep except after performing Witr, and the Duha prayer (mid-morning prayer) while at home and while traveling."