Explanation

The Companion ‘Abdullāh ibn ‘Amr (may Allah be pleased with him) adopted the approach of resolve with regard to fasting and intended to observe it continuously. So the Prophet (may Allah's peace and blessings be upon him) instructed him to fast three days every month, because that equals fasting the entire year in reward. Since a good deed entails a reward ten times as much, the three days shall be 30 days in reward. Therefore, if a person fasts three days every month, it is as though he fasts his entire lifetime.