He saw the Prophet ﷺ undress for entering the state of ihram and perform a ritual bath.
حدثنا عبد الله بن أبي زياد حدثنا عبد الله بن يعقوب المدني عن ابن أبي الزناد عن أبيه
أنه رأى النبي ﷺ تجرد لإهلاله واغتسل
[Its isnad is sahih] and it is agreed upon (by Bukhari and Muslim). Bukhari narrated it in the Book of Knowledge, chapter on mentioning knowledge and issuing rulings in the masjid, no. 133, and Muslim, no. 1182, from the hadith of Ibn Umar, with this wording.On this subject there are also narrations from Ibn Abbas [Bukhari, no. 1524, and Muslim, no. 1181], Jabir bin Abdullah [Muslim, no. 1183], and Abdullah bin Amr [Ahmad, 2/181].