Sahih al-Bukhari 38
صحيح البخاري#38SahihNarrated Abu Huraira: Allah's Messenger (ﷺ) said, "Whoever observes fasts during the month of Ramadan out of sincere faith, and hoping to attain Allah's rewards, then all his past sins will be forgiven
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih al-Bukhari is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 2:31