Musnad Ahmad 26:896

I read to my father: 'Umar ibn Sa'd Abu Dawud narrated to us, Sufyan narrated to us, from Ma'n, from al-Qasim, who said: 'Abdullah and al-Ash'ath differed. One said: "This is for ten," and the other said: "This is for twenty." He said: "Appoint a man between me and you." He said: "You are between me and yourself." He said: "I will judge with what the Messenger of Allah, peace and blessings of Allah be upon him, judged: 'If the two parties to a sale differ, and there is no evidence, then the statement is the statement of the seller, or they both rescind the sale.'" End of the Musnad of 'Abdullah ibn Mas'ud, may Allah be pleased with him.