Musnad Ahmad 1153:872

Bahz narrated to us, Hammam narrated to us, from Qatadah, from Mu'adhah, who said: I asked Aisha, "How many [rak'ahs] did the Messenger of Allah, peace and blessings of Allah be upon him, pray for the Duha prayer?" She said: "Four rak'ahs, and he would add whatever Allah willed."