The Prophet (peace and blessings be upon him) said:
"When a person says to his brother: 'O disbeliever!', then one of the two certainly becomes deserving of it (the verdict of disbelief) (that is, if the other is not truly a disbeliever, the one who said it himself falls under that disbelief)."
Hadith Referenceصحیح الجامع الصغیر / حرف الألف / 709