From Sahih Muslim (Hadith 1668):
Abdullah b. Harith b. Naufal reported:I had been asking about, as I was desirous to find one among people who should inform me, whether the Messenger of Allah (ﷺ) observed the forenoon prayer, but I found none to narrate that to me except Umm Hani, daughter of Abu Talib (the real sister of Hadrat 'Ali), who told me that on the day of the Conquest the Messenger of Allah (ﷺ) came (to our house) after the dawn had (sufficiently) arisen. A cloth was brought and privacy was provided for him (the Holy Prophet). He took a bath and then stood up and observed eight rak'ahs. I do not know whether his Qiyam (standing posture) was longer, or bending or prostration or all of them were of equal duration. She (Umm Hani) further said: I never saw him saying this Nafl prayer prior to it or subsequently. (Al-Muradi narrated on the authority of Yunus that he made no mention of the words:" He informed me)
Reference: Muslim 1668