Musnad Abu Ya'la al-Mawsili 150:25

Ishaq narrated to us, Abd al-Warith bin Sa'id narrated to us, Yazid al-Rishk narrated to us, from Mu'adhah, from Aisha, that she asked her: "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 add whatever Allah willed."