I heard Allah's Messenger (ﷺ) saying like this.
Sahih Muslim Book 13, Hadith 265
← Previous Hadith He who observed the fast of Ramadan and then followed it with six (fasts) of Shawwal. it would be as if he fasted perpetually.

Sahih Muslim Book 13, Hadith 264 Next Hadith → Abu Ayyub reported a hadith like this (through another chain of transmitters).

Sahih Muslim Book 13, Hadith 266