Musannaf Ibn Abi Shayba 13:2069

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Sufyan narrated to us, from Jabir, from ‘Amir, from Zayd, regarding a man who sold a camel to a man and stipulated [the condition of keeping] its head. He said to him: "[It is] the equivalent of the head."