Musannaf Ibn Abi Shayba 7:459

12603. Ibn Numayr narrated to us, from ‘Abdullah ibn Buraydah, from his father, who said: I was sitting with the Prophet, may Allah bless him and grant him peace, when a woman came to him and said: "My mother had a fast of two months upon her; would it suffice for her if I fast on her behalf?" He said: "Yes."