From Sahih Muslim (Hadith 2717):
Abdullah b. Shaqiq reported:I said to'A'isha (Allah be pleased with her): Did the Messenger of Allah (ﷺ) fast for a full month besides Ramadan? She said: I do not know of any month in which he fasted throughout, 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
Reference: Muslim 2717