Imam Malik's Muwatta, Chapter: Hajj

Yahya related to me from Malik from Abdullah ibn Dinar that he used to see Abdullah ibn Umar sacrificing animals two at a time during hajj and one at a time during umra. He said, "I saw him sacrifice an animal during an umra outside the house of Khalid ibn Usayd, where he was staying. I saw him stick his spear in the throat of the animal he was going to sacrifice until the spear came out under its shoulder."