سنن النسائي

It was narrated that 'Abdul-Malik bin'Ubaid said: "We were with Abu 'Ubaidah bin 'Abdullah bin Mas'ud when two men who were involved in a transaction came to him. One of them said: 'I bought it for such and such', and the other said; 'I sold it to him for such and such,' Abu 'Ubaidah said" 'something like this was brought to Ibn Masud, and he said; I was with something like this was brought to him. He told the seller to swear an oath, them he gave the purchaser the choice; If he wished, he could buy it, and if he wished he could cancel (the transaction)''