Jami` at-Tirmidhi 759
جامع الترمذي#759SahihAbu Ayub narrated that :the Messenger of Allah said: "Whoever fasts Ramadan, then follows it with six from Shawwal, then that is (equal in reward) to fasting everyday
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Ahmad Muhammad ShakirHasan Sahih
Al-AlbaniHasan Sahih
Bashar Awad MaaroufHasan Sahih
Zubair Ali ZaiSahih Muslim
In-book reference: 8:78