The Prophet (ﷺ) as saying: If anyone fasts during Ramadan, then follows it with six days in Shawwal, it will be like a perpetual fast.
Sunan Abi Dawud Book 14, Hadith 121
← Previous Hadith I asked or someone asked the Prophet (ﷺ) about perpetual fasting. He replied: You have a duty to your family. Fast during Ramadan and the following month, and every Wednesday and Thursday. You will then have observed a perpetual fast.

Sunan Abi Dawud Book 14, Hadith 120 Next Hadith → The Messenger of Allah (ﷺ) used to fast to such an extent that we thought that he would never break his fast, and he would go without fasting to such an extent that we thought he would never fast. I never saw the Messenger of Allah (ﷺ) fast a complete month except in Ramadan, and I never saw his fast more in any month than in Sha'ban.

Sunan Abi Dawud Book 14, Hadith 122