Musnad Ahmad 32:1932

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, and Hajjaj narrated to us, he said: Shu‘bah narrated to me, he said: I heard Qatadah narrating from Anas ibn Malik, and Yahya ibn Sa‘id narrated to us, from Shu‘bah, Qatadah narrated to us, from Anas: "That the Messenger of Allah, may Allah bless him and grant him peace, used to sacrifice two white-and-black rams with horns. He would mention the name of Allah and magnify Allah, the Mighty and Majestic. I saw him slaughtering them with his own hand, placing his foot—meaning—on their sides."