NASAI-KUBRA 2876:
from Abū Ayyūb —
[Machine] The Messenger of Allah ﷺ said, "Whoever fasts during the month of Ramadan and follows it with six days of Shawwal, it is as if he has fasted for the entire year."
الكبرى للنسائي ٢٨٧٦:
عن ابو ايوب —
قَالَ رَسُولُ اللهِ ﷺ «مَنْ صَامَ رَمَضَانَ وَأَتْبَعَهُ سِتًّا مِنْ شَوَّالٍ فَكَأَنَّمَا صَامَ الدَّهْرَ كُلَّهُ»