Sahih al-Bukhari 1663

Narrated 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-Hijja), 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."