Sahih Muslim 2758
صحيح مسلم#2758SahihAbu Ayyub al-Ansari (Allah be pleased with him) reported Allah's Messenger (ﷺ) as saying:He who observed the fast of Ramadan and then followed it with six (fasts) of Shawwal. it would be as if he fasted perpetually
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih Muslim is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 13:264