Musnad Ahmad 25:427

Isma'il ibn 'Umar narrated to us, Ibn Abi Dhi'b narrated to us, from Shu'bah, from Ibn 'Abbas: That Usamah ibn Zayd was riding behind the Messenger of Allah (may Allah bless him and grant him peace) on the day of 'Arafah. He entered the mountain pass, then he dismounted and poured water, then he performed ablution, and he mounted and did not pray.