Sunan Ibn Majah, Volume 2, Book 7, Hadith 1716 (DarusSalam) ⓘ
It was narrated from Abu Ayyub that the Messenger of Allah said: “Whoever fasts Ramadan then follows it with six days of Shawwal, it is as if he fasted for a lifetime.”
حَدَّثَنَا عَلِيُّ بْنُ مُحَمَّدٍ، حَدَّثَنَا عَبْدُ اللَّهِ بْنُ نُمَيْرٍ، عَنْ سَعْدِ بْنِ سَعِيدٍ، عَنْ عُمَرَ بْنِ ثَابِتٍ، عَنْ أَبِي أَيُّوبَ، قَالَ قَالَ رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ " مَنْ صَامَ رَمَضَانَ ثُمَّ أَتْبَعَهُ بِسِتٍّ مِنْ شَوَّالٍ كَانَ كَصَوْمِ الدَّهْرِ " .
also referenced under: Hadith 1716 (Reference) & Book 7, Hadith 1787 (In-Book)