Musannaf Ibn Abi Shayba 13:816

Abu Bakr narrated to us, he said: Hafs bin Ghiyath narrated to us, from Ash'ath, from al-Hasan, who said: If a man dies and he has a debt upon him, and his heir is not known, then let him place it in the way of Allah. And if he was a Muslim and his heir is not known, then let him give it away as charity on his behalf.