Musnad Ahmad 27:1374

'Affan narrated to us, Sakhr—meaning Ibn Juwayriyah—narrated to us, Nafi' narrated to us, that 'Abdullah ibn 'Umar informed him that the Messenger of Allah (ﷺ) said: "If a man says to his companion, 'O disbeliever,' then it becomes binding upon one of the two. If the one who was addressed is a disbeliever, then he is as he was called; otherwise, it returns upon the one who said it."