Sahih Muslim Book 6, Hadith Number 2560.
Chapter : Completing of the fast on behalf of the dead.
Ibn Buraida (Allah be pleased with him) reported on the authority of his father: A woman came to the Apostle of Allah (may peace be upon him), and the rest of the Hadith is the same, but he said: “Fasting of one month.” This Hadith has been narrated on the authority of Sufyan with the same chain of transmitters in which it is said: “Fasting of two months.”