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.  

عَنْ أَبِي أَيُّوبَ صَاحِبِ النَّبِيِّ ﷺ عَنِ النَّبِيِّ ﷺ قَالَ مَنْ صَامَ رَمَضَانَ ثُمَّ أَتْبَعَهُ بِسِتٍّ مِنْ شَوَّالٍ فَكَأَنَّمَا صَامَ الدَّهْرَ