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: "When a person calls his brother 'O disbeliever,' then one of the two must bear it; either the one who was called a disbeliever is indeed a disbeliever, or else it returns upon the one who said it."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 5824