Musnad Ahmad Hadith 6151

Anas bin Sirin, may Allah have mercy on him, says that I was in the plain of Arafat with Ibn Umar, may Allah be pleased with them both. When he set out, I also started walking with him. He reached the Imam and performed the Zuhr and Asr prayers together with him, then stood with him (for Wuquf). I and some of my companions also participated with them, until when the Imam departed, we also departed with him. When Ibn Umar, may Allah be pleased with them both, reached a narrow path, he made his camel sit down, and we also made our camels sit down. We thought that he wanted to pray, but his servant, who was holding his mount, informed us that Ibn Umar, may Allah be pleased with them both, did not intend to pray. In fact, he remembered that when the Prophet, peace and blessings be upon him, reached this place, he relieved himself. Therefore, he wished to fulfill this Sunnah as well by relieving himself.