Musnad Abu Ya'la al-Mawsili 116:33

Ahmad narrated to us, Abu Dawud narrated to us, he said: Shu'bah and Hisham informed us, from Qatadah, from Anas, who said: The Messenger of Allah (may Allah bless him and grant him peace) sacrificed two white-and-black horned rams. He would mention the name of Allah and say the takbir. And I saw him placing his foot upon their sides.