Musnad Ahmad 1153:2268

'Abd al-Wahhab narrated to us, from Sa'id, from Qatadah, from Mu'adhah, from 'Aishah, that the Prophet (peace and blessings of Allah be upon him): "Used to pray the Duha (forenoon) prayer as four [rak'ahs], and he would increase [them] by whatever Allah willed."