Reward for fasting Ramadan plus six days of Shawwal amounts to fasting a lifetime

He who observed the fast of Ramadan and then followed it with six (fasts) of Shawwal. it would be as if he fasted perpetually.  

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ مَنْ صَامَ رَمَضَانَ ثُمَّ أَتْبَعَهُ سِتًّا مِنْ شَوَّالٍ كَانَ كَصِيَامِ الدَّهْرِ