The Prophet ﷺ had taken him on his mount behind him from Arafat. When he reached the mountain pass, he dismounted and relieved himself. The narrator did not say "he poured out water," but rather, "I poured water for him from a small vessel, and he performed a light ablution."I said to him, "The prayer."He replied, "The prayer is ahead of you."When he reached Muzdalifa, he prayed Maghrib. Then they unloaded their saddles, and afterward he prayed Isha.
أخبرنا الحسين بن حريث قال حدثنا سفيان عن إبراهيم بن عقبة ومحمد بن أبي حرملة عن كريب عن ابن عباس
وكان النبي ﷺ أردفه من عرفة فلما أتى الشعب نزل فبال ولم يقل أهراق الماء قال فصببت عليه من إداوة فتوضأ وضوءا خفيفا فقلت له الصلاة فقال الصلاة أمامك فلما أتى المزدلفة صلى المغرب ثم نزعوا رحالهم ثم صلى العشاء
[Sahih] Ahmad narrated it in 5/200 from Sufyan bin Uyayna, in a shortened form, and it is also in al-Kubra, no. 1579. The hadith has other chains recorded by Bukhari, Muslim, and Baghawi in the Musnad of the Beloved, Son of the Beloved, Usama bin Zayd, no. 2826.