Musnad Ahmad 33:365

'Abd al-Samad narrated to us, Hammad narrated to us, from 'Ali ibn Zayd, from Abu al-Mutawakkil, from Jabir, that he bought a camel for thirteen dinars. The Messenger of Allah (peace and blessings of Allah be upon him) said to him: "For how much did you take it?" He said: "For thirteen dinars." The Messenger of Allah (peace and blessings of Allah be upon him) said to him: "Sell it to me for what you took it for, and you shall have its back [the right to ride it] to Medina."