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