Musnad Ahmad 1153:1332

'Abd al-Razzaq narrated to us, he said: Ma'mar narrated to us, from al-Zuhri, from 'Urwah, from 'Aishah, she said: "The Messenger of Allah, peace and blessings of Allah be upon him, did not perform the Duha prayer." She said: And 'Aishah said: "The Messenger of Allah, peace and blessings of Allah be upon him, would indeed abandon a deed, while he loved to perform it, for fear that the people would adopt it as a sunnah, and thus it would become obligatory upon them." She said: "And he used to love what was light for the people."