Musnad Ahmad 30:478

Abu al-Abbas Muhammad ibn al-Sammak narrated to us, al-Awwam ibn Hawshab narrated to us, he who heard Abu Hurayrah say narrated to me: My intimate friend (Khalil), may Allah bless him and grant him peace, enjoined upon me: fasting three days of every month, performing the Witr prayer before sleeping, and the Duha prayer, for it is the prayer of the oft-returning (al-awwabin).