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