Musnad Ahmad 27:586

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from 'Abdullah ibn Dinar, I heard Ibn 'Umar narrating from the Prophet, may Allah bless him and grant him peace, that he said: "If a man says to his brother: 'O disbeliever,' then it has indeed returned upon one of the two. If it is as he said, [it is so]; otherwise, it returns upon the one who said it."