Salim (may Allah have mercy on him) said that al-Hajjaj asked Sayyiduna Ibn Umar (may Allah be pleased with them both) how one should act at the standing place on the Day of Arafah. Salim (may Allah have mercy on him) said: If you want the Sunnah, then pray early, in the midday heat, on the Day of Arafah. Sayyiduna Ibn Umar (may Allah be pleased with them both) said: He has spoken the truth; they used to combine Zuhr and Asr on the Day of Arafah in accordance with the Sunnah. Ibn Shihab (may Allah have mercy on him) said: I said to Salim (may Allah have mercy on him): Did the Messenger of Allah (peace and blessings be upon him) do that? Salim (may Allah have mercy on him) said: Do they follow anything other than his (peace and blessings be upon him) Sunnah?
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9456