Musnad Ahmad 1018:107

Hashim narrated to us, Layth narrated to us, Yazid ibn Abi Habib narrated to us, from Abu al-Khayr, that a man from the Ansar narrated to him, from the Messenger of Allah, peace and blessings of Allah be upon him, that he laid down his sacrificial animal to slaughter it. The Messenger of Allah, peace and blessings of Allah be upon him, said to the man: "Help me with my sacrificial animal." So he helped him.