Musannaf Ibn Abi Shayba 13:2800

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Isra’il narrated to us, from Jabir, from ‘Amir and Al-Qasim ibn ‘Abd al-Rahman, that they used to say to the buyer: "Establish the evidence that it was sold, and he is a witness who is not denied."