Musannaf Ibn Abi Shayba 13:815

Abu Bakr narrated to us, he said: Ibn Idris narrated to us, from Shu'bah, from Abdullah ibn Hanash, from Ibn Umar, regarding a man who died while having a debt upon him, and the owner of the debt was not known. He ordered that it be given away as charity on his behalf, equivalent to that debt.