Musnad Ahmad 32:2028

Abdullah narrated to us, Abdullah bin Sa’d bin Ibrahim al-Zuhri Abu al-Qasim narrated to me, he said: My paternal uncle Ya’qub bin Ibrahim narrated to me, from Sharik, from Shu’bah bin al-Hajjaj, from Qatadah, from Anas, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, used to sacrifice two horned, black-and-white rams. He would mention the name of Allah and say the takbir. And I indeed saw him slaughtering them with his own hand, placing his foot upon their sides."