Musnad Ahmad 27:297

Ya'la ibn 'Ubayd narrated to us, Fudayl—meaning Ibn Ghazwan—narrated to us, from Nafi', from Ibn 'Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Any man who calls a man a disbeliever, then if he is as he said [it is so], and otherwise, he has returned with the disbelief."