Musannaf Ibn Abi Shaybah

Hazrat Ata says that if a person buys some goods and then finds a defect in them, the buyer will need to provide testimony that this defect was present while the goods were with the seller. If testimony is provided, then fine; otherwise, the seller will be made to swear an oath that he was not aware of the defect. Hazrat Amr bin Dinar says that the oath should be regarding knowledge.