Musnad Ahmad 1153:1433

I read to Abd al-Rahman: Malik, from Ibn Shihab, from Urwah ibn al-Zubayr, from Aisha, the wife of the Prophet, may Allah bless him and grant him peace, that she said: "The Messenger of Allah, may Allah bless him and grant him peace, never prayed the Duha prayer, yet I pray it. And the Messenger of Allah, may Allah bless him and grant him peace, would indeed leave a deed while he loved to perform it, for fear that the people might perform it, and thus it would become obligatory upon them."