Musnad Ahmad 30:3212

Muhammad ibn Ja‘far narrated to us, he said: Sa‘id narrated to us, from Qatadah, from al-Hasan, from Abu Hurayrah, 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, whether in travel or at home: "Fasting three days of every month, sleeping after performing Witr, and the two rak‘ahs of Duha." He said: Then al-Hasan became confused, and he attributed the two rak‘ahs of Duha to the Ghusl (ritual bath) on Friday.