Musnad Ahmad 27:1691

Yazid ibn Harun narrated to us, 'Abd al-Malik informed us, from Anas ibn Sirin, who said: I was with Ibn 'Umar at 'Arafat. When it was time for the departure, I departed with him until he came to the Imam, and he prayed the Dhuhr and 'Asr prayers with him. Then he stood with him, and I and some companions of mine stood with him, until the Imam departed, and we departed with him. Until we reached the narrow pass before the two mountain passes (al-Ma'zamayn), he made his mount kneel, and we made our mounts kneel. We thought that he wanted to pray. His servant, who was holding his mount, said: "He does not want to pray, but he remembered that when the Prophet, peace and blessings of Allah be upon him, reached this place, he relieved himself, so he likes to relieve himself."