Majma al-Zawaid — Hadith 812

Anas bin Sirin narrates: I was with Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) at Arafat. When it was time to depart, I departed with him, until the imam came, and Sayyiduna Abdullah (may Allah be pleased with him) offered the Zuhr and Asr prayers behind him. Then he, I and my companions stayed until the imam departed, and we departed with them, until he reached a narrow place before al-Ma'zimayn. He made his mount kneel, and we made our mounts kneel too, thinking that perhaps he intended to pray. His slave, who was holding his mount, told us: He does not intend to pray; rather, he has mentioned that when the Prophet (peace and blessings be upon him) reached this place, he (peace and blessings be upon him) relieved himself here, so Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) likes to relieve himself here.
Ahmad recorded it, and its narrators are the narrators of the Sahih.