The Prophet ﷺ as saying: If anyone fasts during Ramadan, then follows it with six days in Shawwal, it will be like a perpetual fast.

عَنِ النَّبِيِّ ﷺ قَالَ مَنْ صَامَ رَمَضَانَ ثُمَّ أَتْبَعَهُ بِسِتٍّ مِنْ شَوَّالٍ فَكَأَنَّمَا صَامَ الدَّهْرَ