Hadith 5:90
Abu Ayyub al-Ansari told that God’s messenger said, “If anyone fasts during Ramadan, then follows it with six days in Shawwal, it will be like a perpetual fast.” Muslim transmitted it.
وَعَنْ أَبِي أَيُّوبَ الْأَنْصَارِيِّ أَنَّهُ حَدَّثَهُ أَنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ: «مَنْ صَامَ رَمَضَانَ ثُمَّ أَتْبَعَهُ سِتًّا مِنْ شَوَّال كَانَ كصيام الدَّهْر» . رَوَاهُ مُسلم
Original Reference: Mishkat al-Masabih 2047