Sahih Bukhari Volume 2, Book 26, Hadith Number 724.
Narated By Salim bin ‘Abdullah bin ‘Umar : ‘Abdul-Malik bin Marwan wrote to Al-Hajjaj that he should follow ‘Abdullah bin ‘Umar in all the ceremonies of Hajj. So when it was the Day of ‘Arafat (9th of Dhul-Hajja), and after the sun has deviated or has declined from the middle of the sky, I and Ibn ‘Umar came and he shouted near the cotton (cloth) tent of Al-Hajjaj, “Where is he?” Al-Hajjaj came out. Ibn ‘Umar said, “Let us proceed (to ‘Arafat).” Al-Hajjaj asked, “Just now?” Ibn ‘Umar replied, “Yes.” Al-Hajjaj said, “Wait for me till I pour water on me (i.e. take a bath).” So, Ibn ‘Umar dismounted (and waited) till Al-Hajjaj came out. He was walking between me and my father. I informed Al-Hajjaj, “If you want to follow the Sunna today, then you should shorten the sermon and then hurry up for the stay (at ‘Arafat).” Ibn ‘Umar said, “He (Salim) has spoken the truth.”