Musnad Abu Ya'la al-Mawsili 152:337

Ibrahim ibn al-Hajjaj narrated to us, Hammad ibn Zayd narrated to us, from Aban ibn Taghlib, from al-Qasim ibn Abd al-Rahman: That Ibn Mas'ud sold some slaves from the state-owned slaves to al-Ash'ath. Then he came to him to demand payment, and they disagreed regarding the price. Ibn Mas'ud said: "Are you satisfied that I judge between myself and you with the judgment of the Messenger of Allah (may Allah bless him and grant him peace)? When the two parties to a sale disagree, the statement is what the seller says, or they rescind the transaction."