Musannaf Ibn Abi Shayba 35:73

Ibn Numayr narrated to us, from ‘Abdullah ibn ‘Ata’, from Ibn Buraydah, from his father, who said: I was sitting with the Prophet (peace be upon him) when a woman came to him and said: "My mother had a fast of two months upon her; shall I fast on her behalf?" He said: "Fast on her behalf." He said: "Have you considered if your mother had a debt, and you paid it off, would it suffice for her?" She said: "Yes." He said: "Then fast on her behalf."