Musnad Ahmad 941:8

Sufyan narrated to us, from Ibrahim ibn 'Uqbah, from Kurayb, from Ibn 'Abbas, who said: Usamah ibn Zayd informed me that the Prophet, peace and blessings of Allah be upon him, let him ride behind him from 'Arafah. When he reached the mountain pass, he dismounted and urinated—and he did not say "he poured water"—so I poured [water] for him, and he performed a light ablution. I said: "The prayer?" He said: "The prayer is ahead of you." He said: Then he came to Muzdalifah and prayed the Maghrib [prayer]. Then they unloaded their baggage, and I helped them, then he prayed the 'Isha [prayer].