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