It was narrated from Abu Dharr that the Messenger of Allah (ﷺ) said: “Whoever fasts three days in every month, that is fasting for a lifetime.” Then, in testimony of that, Allah revealed: “Whoever brings a good deed shall have ten time the like thereof to his credit.” [6:160] So one day is equivalent to ten (in reward).

It was narrated from ‘Abdul-Malik bin Minhal, from his father that: The Messenger of Allah (ﷺ) used to enjoin fasting the bright days – the thirteenth, fourteenth and fifteenth (when the moon is full). He said: “It is like fasting for a lifetime.”

It was narrated from Mu’adhah Al-‘Adawiyyah that ‘Aishah said: “The Messenger of Allah (ﷺ) used to fast three days of each month.” I said: “Which were they?” She said: “He did not care which days they were.”