the Messenger of Allah said: "Whoever fasts Ramadan, then follows it with six from Shawwal, then that is (equal in reward) to fasting everyday."  

قَالَ النَّبِيُّ ﷺ مَنْ صَامَ رَمَضَانَ ثُمَّ أَتْبَعَهُ سِتًّا مِنْ شَوَّالٍ فَذَلِكَ صِيَامُ الدَّهْرِ