´It was narrated that Abu Hurairah said:` " I heard the Messenger of Allah say: 'The month of patience ( Ramadan) and three days of each month is fasting for a lifetime."' (Sahih).
Explanation & Benefits
Hafiz Muhammad Ameen
The fasts of Ramadan are obligatory. As for three fasts in every (other) month, in terms of reward, they are equal to fasting the entire month. Ramadan has been called the month of patience because fasting itself is (an act of) patience: patience from eating and drinking, patience from desires, and patience from quarrelling and foul language.
Source: Sunan Nasa'i: Translation and Benefits by Shaykh Hafiz Muhammad Amin Hafizullah, Page: 2410