Reference: Fataawa al-Lajnah ad-Daa.imah lil-Buhooth al-‘Ilmiyyah wal-Iftaa., Fatwa No.7306
Fataawa Ramadhaan – Volume 2, Page 696, Fatwa No.702
Question: Someone used to fast the six [days] of Shawwaal. If he [then] becomes ill or something prevents him from fasting or he just becomes lazy from fasting in a particular year, is there any sin upon him, because we have heard that whoever fasts them (six days of Shawwaal) in [any particular] year then it is obligatory for him not to leave them (i.e. continue to fast them every year)?
Response: The fasting of the six [days] of Shawwaal after the day of ‘Eed [al-Fitr] is a Sunnah, and it is not obligatory upon whoever fasts them once or more [than once] to continue fasting them, and there is no sin upon whoever leaves off fasting them.
And with Allaah lies all success and may Allaah send prayers and salutations upon our Prophet (sal-Allaahu ‘alayhi wa sallam) and his family and his companions.