Is fasting the six days of Shawwal obligatory every year after having fasted them once?

Fasting six days of Shawwal after Ramadan is a recommended Sunnah according to most scholars, based on the Prophet Muhammad’s (peace be upon him) saying: "Whoever fasts Ramadan and then follows it with six days of Shawwal, it is as if he has fasted the entire year." If one fasts them in a given year, it does not become obligatory upon him in subsequent years; rather, it remains a recommended act always, unless a Muslim vows to fast them, in which case it becomes obligatory due to the vow.