Musnad Ahmad 1153:621

Bahz narrated to us, he said: Hammam narrated to us, from Qatadah, from Mu'adhah, she said: I asked Aisha, "How many [rak'ahs] did the Messenger of Allah, may Allah bless him and grant him peace, pray for the Duha prayer?" She said: "Four rak'ahs, and he would increase whatever Allah the Mighty and Majestic willed."