Musnad Ahmad 1153:907

'Affan narrated to us, he said: Shu'bah narrated to us, he said: Yazid al-Rishk informed me, from Mu'adhah, from 'Aishah, that she asked her: "Did the Messenger of Allah, peace and blessings of Allah be upon him, pray the Duha (forenoon) prayer as four [rak'ahs]?" She said: "Yes, four, and he would increase whatever Allah willed."