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 andgrant him peace, said, "I never once saw the Messenger of Allah, mayAllah bless him and grant him peace, doing the voluntary prayer ofduha, but I myself do it. Sometimes the Messenger of Allah, may Allahbless him and grant him peace, would refrain from a practice that heloved to do, fearing that people would do the same and it would becomefard for them ."
USC-MSA web (English) reference: Book 9, Hadith 32
Arabic reference: Book 9, Hadith 361