Musnad Ahmad 30:553

'Abd al-Razzaq narrated to me, Ma'mar narrated to us, from Qatadah, from al-Hasan, from Abu Hurayrah, who said: The Prophet, may Allah bless him and grant him peace, enjoined upon me three things that I will not abandon, whether at home or while traveling: sleeping after performing the Witr prayer, fasting three days of every month, and the two rak'ahs of Duha. He said: Then al-Hasan became uncertain afterward, and he placed "performing ghusl on Friday" in place of "the Duha [prayer]."