Musnad Ahmad 26:893

It was read to my father: Yahya ibn Sa'id narrated to us, from Ibn 'Ajlan, who said: 'Awn ibn 'Abdullah narrated to me, from Ibn Mas'ud, who said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "If the two parties to a sale disagree, then the statement is what the seller says, and the buyer has the choice."