The Prophet ﷺ prayed Zuhr, then mounted his riding camel, and when he ascended the hill of Bayda, he entered the state of ihram.
حدثنا روح حدثنا أشعث عن الحسن
أن النبي ﷺ صلى الظهر ثم ركب راحلته فلما علا جبل البيداء أهل
Its isnad is sahih. It is found in Bukhari with similar wording in 3/407, no. 1546, Fath al-Bari, in the book of Hajj, chapter on Zulhulayfa, and in Abu Dawud 2/151, no. 1775, in the book of rites, on the time of ihram, and in Nasai 5/127, no. 2662, in the book of rites, on Bayda.