Sahih Muslim 215
صحيح مسلم#215SahihIt is reported on the authority of Ibn 'Umar that the Apostle (may peace and blessings be upon him) observed:When a man calls his brother an unbeliever, it returns (at least) to one of them
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih Muslim is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 1:121