Musnad Ahmad 26:894

I read to my father: Waki’ narrated to us, from al-Mas’udi, from al-Qasim, from ‘Abdullah ibn Mas’ud, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "If the two parties to a sale disagree, and there is no evidence between them, then the statement is what the owner of the merchandise says, or they shall rescind the transaction."