Sunan Ibn Majah
1716
Hasan Sahih+2

It was narrated from Abu Ayyub that the Messenger of Allah (ﷺ) said:“Whoever fasts Ramadan then follows it with six days of Shawwal, it is as if he fasted for a lifetime.”

Hasan SahihAl-Albani
Hasan SahihMuhammad Fouad Abd al-Baqi