Urwah narrates from Sayyidah Aishah (may Allah be pleased with her) that she said: I never saw the Prophet (peace and blessings be upon him) offering the voluntary Duha prayer, but I do offer it; and the Messenger of Allah (peace and blessings be upon him) would leave a deed even though he loved to do it, fearing that people would act upon it and it would be made obligatory upon them.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4913