Musnad Ahmad 1153:546

'Ali ibn 'Ayyash narrated to us, he said: Shu'ayb narrated to us, from al-Zuhri, he said: And 'Urwah ibn al-Zubayr informed me, that 'Aishah, the wife of the Prophet, may Allah bless him and grant him peace, said: "By Allah, the Messenger of Allah, may Allah bless him and grant him peace, never performed the Duha prayer, and I perform it." And she said: "Indeed, the Messenger of Allah, may Allah bless him and grant him peace, used to 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. And the Messenger of Allah, may Allah bless him and grant him peace, used to love that which was light for the people regarding the obligatory acts."