Musannaf Ibn Abi Shayba 13:898

Abu Bakr narrated to us, he said: Abbad bin al-Awwam narrated to us, from Hisham, from Ibn Sirin, from Shurayh, that he used to say regarding two parties in a sale, if they disagree while the item of sale is still present in its essence: He would ask them for evidence. If one of them produced evidence, he would be given [the ruling] based on his evidence. If neither of them had evidence, he would require them both to swear an oath. If they both came with it [the oath], the sale would be rescinded. If they both refused to swear, the sale would be rescinded. If one of them swore and the other refused, the one who swore would be given [the ruling]. If the item of sale was not present in its essence, or he said it has been consumed, the seller is required to provide evidence, and the oath is upon the buyer.