The Messenger of Allah ﷺ said, "When a man declares another man to be a disbeliever, one of the two is indeed a disbeliever."
حدثنا وكيع عن فضيل بن غزوان عن نافع
قال رسول الله ﷺ أيما رجل كفر رجلا فأحدهما كافر
No references are available.
Its isnad is sahih, and it is a repetition of the one before it.