From Sahih Bukhari (Hadith 4187):
Abdullah bin `Umar added:"The people were along with the Prophet (ﷺ) on the day of Al-Hudaibiya spreading in the shade of the trees. Suddenly the people surrounded the Prophet (ﷺ) and started looking at him." `Umar said, "O `Abdullah! Go and see why the people are encircling Allah's Apostle and looking at him." `Abdullah bin `Umar then saw the people giving the Pledge of allegiance to the Prophet. So he also gave the Pledge of allegiance and returned to `Umar who went out in his turn and gave the Pledge of allegiance to the Prophet
Reference: Bukhari 4187