I heard the Messenger of Allah ﷺ say, "Three things are obligations upon me but voluntary acts for you, the witr prayer, the sacrifice, and the duha prayer."
حدثنا شجاع بن الوليد عن أبي جناب الكلبي عن عكرمة
سمعت رسول الله ﷺ يقول ثلاث هن علي فرائض وهن لكم تطوع الوتر والنحر وصلاة الضحى
No references are available.
Its isnad is daif. Abu Janab Kalbi is Yahya bin Abu Hayya, whose weakness has already been noted at 1136. The hadith was narrated by Hakim 1:300 and Daraqutni 171, both by way of Shuja bin Walid, but in Daraqutni it reads "the two rakahs of Fajr" instead of "the prayer of Duha." Dhahabi said, "It is gharib munkar, and Yahya was declared daif by Nasai and Daraqutni." See Nasb Raya 2:115, and see what preceded at 1261.