Musnad Ahmad 32:768

Ya'la narrated to us, Mis'ar narrated to us, from Bukayr ibn al-Akhnas, who said: I heard Anas ibn Malik saying: A sacrificial camel—or a gift—passed by the Prophet, may Allah bless him and grant him peace, and he said to its owner: "Ride it." He said: "It is a sacrificial camel—or a gift." He said: "Even so."