Musnad Ahmad 26:895

I read to my father: Ibn Mahdi narrated to us, he said: Sufyan narrated to us, from Ma'n, from al-Qasim, from Abdullah, from the Prophet, peace and blessings of Allah be upon him, who said: "If the two parties to a sale disagree, and the commodity is as it was, then the statement is what the seller says, or they both rescind the transaction."