Musannaf Ibn Abi Shayba 13:897

Abu Bakr narrated to us, he said: Hushaym narrated to us, from Isma'il ibn Salim, from al-Sha'bi, who said: If the two parties to a sale disagree, and there is no evidence between them, and the item of sale remains in its original state, then the statement is the statement of the seller, or they rescind the sale. If the item of sale has been consumed, then the statement is the statement of the buyer, and the burden of proof is upon the seller.