Musnad Ahmad 941:20

Ya'qub narrated to us, my father narrated to us, from Muhammad ibn Ishaq, Ibrahim ibn 'Uqbah narrated to me, from Kurayb, the freed slave of 'Abdullah ibn 'Abbas, from Usamah ibn Zayd, who said: I was riding behind the Messenger of Allah (peace and blessings of Allah be upon him) on the evening of 'Arafah. When the sun set, the Messenger of Allah (peace and blessings of Allah be upon him) departed. When he heard the clamor of the people behind him, he said: "Gently, O people! Upon you is tranquility, for righteousness is not in rushing." He said: When the people crowded upon the Messenger of Allah (peace and blessings of Allah be upon him), he would quicken his pace, and when he found an opening, he would trot, until he passed by the mountain pass where many people claim he prayed. He descended there and urinated—he did not say "poured water" as you say—then I brought him the water skin and he performed ablution. Then I said: "The prayer, O Messenger of Allah?" He said: "The prayer is ahead of you." He said: So the Messenger of Allah (peace and blessings of Allah be upon him) mounted and did not pray until he reached Muzdalifah. He descended there and combined the two prayers: Maghrib and the last 'Isha'.