Question: It is written in Islamic books that one who fasts for a month in Ramadan, since whoever does a good deed shall have at least ten times its reward, is considered as if one fasted for 300 days and who also fasts for six days in Shawwal is considered as if one fasted 60 days. That is, it is written that one is considered to have fasted the entire year. How can Ramadan fasts, which are fard, be equated with the Shawwal fasts, which are voluntary?
It is also written in Islamic books that one who fasts for three days every month is considered to have fasted all the year round. Ramadan fasts, which are fard, are not compared with voluntary fasts in these statements. It is not stated that such a person has fasted the whole year. What is stated is that such a person will be judged to have fasted. Otherwise, if one observes voluntary fasts all one's life, one still cannot earn the rewards given in return for a fast observed in the holy month of Ramadan. If one misses a fast-day in Ramadan without a valid excuse, one still cannot earn the rewards of this single fast-day of Ramadan even if one observes voluntary fasts all one's life. What is more, if one observes voluntary fasts after making up for the fast-day missed in Ramadan, one still cannot earn the rewards that are given in Ramadan in return for a single fast. When one makes up for a missed fast of Ramadan later, one is only absolved of the debt; one can in no way earn rewards that are given when it is observed in Ramadan. As a matter of fact, a hadith-i sharif says:
(If a person does not fast for a single day in Ramadan, he cannot earn the rewards of this single day, even if he fasts all the year round instead of it.) [Tirmidhi]