Musannaf Ibn Abi Shayba 13:2895

Abu Bakr narrated to us, he said: Sufyan narrated to us, from Ubaydah, from Abu Salih, from al-Sha’bi, who said: Whoever guarantees a debt on behalf of a man without him ordering him to do so, and then he pays it on his behalf, then the one on whose behalf the guarantee was made owes him nothing. It is merely a burden that he took upon himself.