Musnad Ahmad 32:1379

Hashim narrated to us, Shu'bah narrated to us, he said: Qatadah informed me, he said: I heard Anas ibn Malik say—I said: Did you hear it from him? He said: Yes—he said: "The Messenger of Allah (peace and blessings of Allah be upon him) used to sacrifice two horned, white-and-black rams. He would mention the name of Allah and say the takbir, and I saw him slaughtering them with his own hand, placing his foot upon their sides."