Musnad Ahmad 33:122

Waki’ narrated to us, from Shu’bah, from Muharib ibn Dithar, from Jabir ibn Abdullah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) bought a camel from me. He weighed the price for me, and he gave me extra. He said: "Have you prayed? Pray two rak'ahs."