Musnad Ahmad 478:6

Ya'qub narrated to us, my father narrated to us, from Salih. Ibn Shihab said: Ja'far ibn 'Amr ibn Umayyah narrated to me, that his father said: "I saw the Messenger of Allah (peace and blessings of Allah be upon him) cutting from the shoulder of a sheep. Then he was called to prayer, so he threw down the knife and did not perform ablution."