Abu Dawud Book 4, Hadith Number 1909.

Narated By Abdullah ibn Umar : When al-Hajjaj killed Ibn Zubayr, he sent a message to Ibn Umar asking him: At which moment the Apostle of Allah (PBUH) used to proceed (to Arafat) this day? He replied: When it happens so, we shall proceed. When Ibn Umar intended to proceed, the people said: The sun did not decline. He (Ibn Umar) asked: Did it decline? They replied: It did not decline. When they said that the sun had declined, he proceeded.