Calling another a Kafir
Tuesday, July 10th, 2007 Sahih Bukhari
Volume 8, Book 73, Number 125d:
Narrated Abu Huraira (radiallahu anho):
Allah’s Apostle (salallahu alaihi wasallam) said, “If a man says to his brother, O Kafir (disbeliever)!’ Then surely one of them is such (i.e., a Kafir). ”