حَدَّثَنَا أَبِي أَيُّوبَ الأَنْصَارِيِّ أن رسول الله ﷺ قال:
"مَنْ صَامَ رَمَضَانَ ثُمَّ أَتْبَعَهُ سِتًّا مِنْ شَوَّالٍ كَانَ كَصِيَامِ الدَّهْرِ"
Abu Ayyub al-Ansari (Allah be pleased with him) reported Allah's Messenger ﷺ as saying: He who observed the fast of Ramadan and then followed it with six (fasts) of Shawwal. it would be as if he fasted perpetually.
Sahih Muslim 1164a