Musnad Ahmad 762:1

Yahya ibn Sa'id narrated to us, from Thawr, who said: Rashid ibn Sa'd narrated to me, from 'Abdullah ibn Luhay, from 'Abdullah ibn Qurt, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "The greatest of days in the sight of Allah is the Day of Sacrifice (Yawm al-Nahr), then the Day of Resting (Yawm al-Qarr)." Five or six camels were brought near to the Messenger of Allah (peace and blessings of Allah be upon him) for him to sacrifice them, and they began to crowd toward him, [competing as to] which of them he would begin with. When their sides had fallen [after being slaughtered], he said a quiet word that I did not understand. I asked someone who was next to me: "What did he say?" They said: "He said: 'Whoever wishes may take a piece.'"