Sahih Muslim Book 6, Hadith Number 2559.

Chapter : Completing of the fast on behalf of the dead.

‘Abdullah b. Buraida (Allah be pleased with him) reported on the authority of his father: I was sitting with the Apostle of Allah (may peace be upon him); the rest of the Hadith is the same but with this variation that the (the narrator) said: “Fasts of two months.”