Abu Ayyoob (may Allaah be pleased with him) reported that the Messenger of Allaah (peace and blessings of Allaah be upon him) said:
“Whoever fasts Ramadaan and follows it with six days of Shawwaal, it will be as if he fasted for a lifetime.”
(Narrated by Muslim, Abu Dawood, al-Tirmidhi, al-Nisaa’i and Ibn Maajah).
Imam Ahmad said:
‘The Prophet ﷺ said ‘6 days of shawwal’, so whoever fasts 6 days, whether consecutively or separately.’