Musannaf Ibn Abi Shayba 13:2944

Abu Bakr narrated to us, he said: Sufyan ibn ‘Uyaynah narrated to us, from Abu Farwah, who heard Ibn Abi Layla say: Umar said: "Whatever dirhams have been debased (counterfeited) for him, he should not swear to the people that they are good. Rather, let him take them out to the market and say: 'Who will sell me these debased dirhams for the equivalent of a garment or some need of his needs?'"