Sunan Abi Dawud 2433
سنن أبي داود#2433SahihNarrated Abu Ayyub: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
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Al-AlbaniSahih
Muhammad Muhyi Al-Din Abdul HamidSahih
Zubair Ali ZaiSahih Muslim (1164)
In-book reference: 14:121