Musnad Ahmad 27:422

Yazid ibn Harun narrated to us, Sufyan—meaning Ibn Husayn—informed us, from al-Hakam, from Mujahid, who said: We were with Ibn Umar on a journey, and he passed by a place and turned away from it. He was asked, "Why did you do that?" He said, "I saw the Messenger of Allah, may Allah bless him and grant him peace, do this, so I did it."