Musnad Ahmad 32:1806

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