Reference: Siyar A’laam an-Nubalaa v12 p391
narrates:
A man came to Abu ‘Abdillaah al-Bukhaaree and said: ‘O Abaa ‘Abdillaah, such-and-such has said that you are a disbeliever!!’
He replied:
The Prophet ﷺ said:
«If a man says to his brother, ‘O disbeliever!’ Then surely one of them is such (i.e. a disbeliever)»
