It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "If a person says to his brother, 'O disbeliever,' then surely one of them returns as a disbeliever."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 5259