Sahih Bukhari Volume 3, Book 31, Hadith Number 174.
Narated By Ibn Abbas : A man came to the Prophet and said, “O Allah’s Apostle! My mother died and she ought to have fasted one month (for her missed Ramadan). Shall I fast on her behalf?” The Prophet replied in the affirmative and said, “Allah’s debts have more right to be paid.” In another narration a woman is reported to have said, “My sister died…”
Narrated Ibn ‘Abbas: A woman said to the Prophet “My mother died and she had vowed to fast but she didn’t fast.” In another narration Ibn ‘Abbas is reported to have said, “A woman said to the Prophet, “My mother died while she ought to have fasted for fifteen days.”