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 ﷺ and his family and his companions.