“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)
سَمِعْتُ رَسُولَ اللهِ ﷺ يَقُولُ مَنْ صَامَ رَمَضَانَ ثُمَّ أَتْبَعَهُ سِتًّا مِنْ شَوَّالٍ فَذَاكَ صِيَامُ الدَّهْرِ