Musnad Ahmad 1153:728

Abu Sa’id narrated to us, he said: ‘Uthman ibn ‘Abd al-Malik Abu Qudamah al-‘Umari narrated to us, he said: ‘A’ishah bint Sa’d narrated to us, from Umm Dharr, she said: I saw ‘A’ishah praying the Duha (forenoon) prayer, and she was saying: "I never saw the Messenger of Allah, may Allah bless him and grant him peace, pray it except as four rak’ahs."