Question: If a person, who is used to fasting the six days of Shawwaal regularly, becomes sick, hindered, or feels lazy to fast them in one of the years, is this a sin? We hear that anyone who fasts them regularly should not give up doing so.
Answer: Fasting the Six Days of Shawwaal after the day of `Eed-ul-Fitr (the Festival of Breaking the Fast) is an act of Sunnah (whatever is reported from the Prophet). It is not obligatory upon anyone who fasts them once or more to continually fast them. It is not a sin to abstain from fasting them... read more here.