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