Abu Dawud Book 4, Hadith Number 1948.

Narated By Sarra’ daughter of Nabhan : She was mistress of a temple in pre-Islamic days. She said: The prophet (PBUH) addressed us on the second day of sacrifice (yawm ar-ru’us) and said: Which is this day? We said: Allah and His Apostle are better aware. He said: Is this not the middle of the tashriq days?