Imam Malik's Muwatta, Chapter: Shortening the Prayer

Yahya related to me from Malik from Ibn Shihab from Urwa ibn az-Zubayr that A'isha, the wife of the Prophet, may Allah bless him and grant him peace, said, "I never once saw the Messenger of Allah, may Allah bless him and grant him peace, doing the voluntary prayer of duha, but I myself do it. Sometimes the Messenger of Allah, may Allah bless him and grant him peace, would refrain from a practice that he loved to do, fearing that people would do the same and it would become fard for them."