Musnad Ahmad 1153:1105

Yazid narrated to us, he said: Hammam ibn Yahya informed us, from Qatadah, from Mu'adhah, from Aisha: "The Prophet, may Allah bless him and grant him peace, used to pray the Duha (forenoon) prayer as four [rak'ahs], and he would increase [by] whatever Allah willed."