Sahih Muslim
The Book of Fasting (Kitab Al-Sawm)
Book Number: 006 / Hadith Number: 2576
Abdullah b. Shaqiq reported: I said to'A'isha (Allah be pleased with her): Did the Apostle of Allah (may peace be upon him) observe fast for full one month besides Ramadan? She said: I do not know of any month in which he fasted through- out, but that of the month of Ramadan and (the month) in which he did not fast at all, till he ran the course of his life.