Sahih Muslim
The Book of Fasting (Kitab Al-Sawm)
Book Number: 006 / Hadith Number: 2600
Mu'adha al-'Adawiyya reported that she asked 'A'isha, the wife of the Apostle of Allah (may peace be upon him), whether the Messenger of Allah (may peace be upon him) observed fasts for three days during every month. She said: Yes I said to her: Which were (the particular) days of the month on which he observed fast? She said: He was not particular about the days of the month on which to observe fast.