'Abdullah ibn 'Umar reported that the Messenger of Allah, may Allah
bless him and grant him peace, said, "When someone says to another, 'Unbeliever!'
then one of them is an unbeliever. If the one to whom he says it is an
unbeliever, he has spoke the truth. If that is not the case, then the one
who said it has brought down disbelief on himself."