Sunnah.com reference : Book 5, Hadith 32English translation : Book 5, Hadith 701Arabic reference : Book 5, Hadith 681
Abu Aiyub Al-Ansari (RAA) narrated that The Messenger of Allah (ﷺ) said:
"Whoever fasts during the month of Ramadan and then follows it with six days of Shawwal will be (rewarded) as if he had fasted the entire year." Related by Muslim.
وَعَنْ أَبِي أَيُّوبَ اَلْأَنْصَارِيِّ ‏- رضى الله عنه ‏- أَنَّ رَسُولَ اَللَّهِ ‏- صلى الله عليه وسلم ‏-قَالَ: { مَنْ صَامَ رَمَضَانَ, ثُمَّ أَتْبَعَهُ سِتًّا مِنْ شَوَّالٍ كَانَ كَصِيَامِ اَلدَّهْرِ }  رَوَاهُ مُسْلِمٌ  1‏ .‏‏1 ‏- صحيح.‏ رواه مسلم ( 1164 )‏.‏