The Messenger of Allah ﷺ said, "Fasting Ramadan equals ten months, and fasting the six days equals two months, so that completes the fasting of an entire year," meaning Ramadan and six days after it.
أن رسول الله ﷺ قال: صيام رمضان بعشر أشهر، وصيام الستة أيام بشهرين، فذلك صيام السنة يعني رمضان وستة أيام بعده.
Ibn Majah (1715), Imam Ahmad (22412), Ibn Khuzayma (2115), Ibn Hibban (3635), Tahawi in his Mushkil (2348), and Bayhaqi (4/293) all narrated it, all of them through Yahya bin Harith Dhamari, from Abu Asma Rahbi, from Thawban, who mentioned it.The wording given is that of Ibn Khuzayma. Ibn Majah's wording is, "Whoever fasts six days after breaking the fast, that completes the year. Whoever comes with a good deed will have ten times its like."Its chain of transmission is sahih. Abu Asma's name is Amr bin Marthad, and he is trustworthy.