Musannaf Ibn Abi Shayba 5:781

Waki’ narrated to us, from Sufyan, from ‘Abdullah ibn Muhammad, from Ziyad ibn Hudayr, who said: I was with my grandfather, and a Christian passed by with a horse whose value was twenty thousand. He said to him: "If you wish, I will give you two thousand, and if you wish, you may take the horse and we will give you its value of eighteen thousand."