Musnad Ahmad 1153:1345

'Abd al-Razzaq narrated to us, he said: Ibn Jurayj informed us, he said: Ibn Shihab narrated to me, from 'Urwah, that 'Aishah used to say: "The Messenger of Allah, peace and blessings of Allah be upon him, did not pray the Duha prayer." She said: And 'Aishah used to pray it, and she used to say: "Indeed, the Messenger of Allah, peace and blessings of Allah be upon him, used to abandon a deed for fear that the people would adopt it as a sunnah, and thus it would become obligatory upon them."