Sahih al-Bukhari 2936
صحيح البخاري#2936SahihNarrated `Abdullah bin `Abbas:Allah's Messenger (ﷺ) wrote a letter to Caesar saying, "If you reject Islam, you will be responsible for the sins of the tillers (i.e. your people)
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: 56:149