Sahih Muslim
The Book of Fasting (Kitab Al-Sawm)
Book Number: 006 / Hadith Number: 2559
'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."