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." (AR)
English reference (AR): Vol. 1, Book 7, Hadith 1716 (AR)
Arabic reference (AR): Book 7, Hadith 1787 (AR)