It wasnarrated from Abu Ayyub that the Messenger of Allah (ﷺ)said:
"Whoever fasts Ramadan then follows it with six days of Shawwal,itis as if he fasted for a lifetime." (PT)
English reference (PT): Vol. 1, Book 7, Hadith 1716 (PT)
Arabic reference (PT): Book 7, Hadith 1787 (PT)