Musnad Ahmad 32:949

Waki' narrated to us, Mis'ar narrated to us, from Bukayr ibn al-Akhnas, he said: I heard Anas ibn Malik saying: A sacrificial animal — or a camel for sacrifice — passed by the Prophet, peace and blessings of Allah be upon him, and he said: "Ride it." He said: "O Messenger of Allah, it is a sacrificial animal — or a camel for sacrifice." He said: "Even so."