“Whoever fasts Ramadan then follows it with six days of Shawwal, it is as if he fasted for a lifetime.” (Using translation from Ibn Mājah 1716)
قَالَ رَسُولُ اللهِ ﷺ مَنْ صَامَ رَمَضَانَ ثُمَّ أَتْبَعَهُ سِتًّا مِنْ شَوَّالٍ فَذَلِكَ صِيَامُ الدَّهْرِ