It is narrated from Sayyiduna Thabit (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "There is no vow in what a person does not own; and cursing a believer is like killing him; and whoever kills himself with something in this world will be punished with it on the Day of Resurrection; and whoever swears falsely by a religion other than Islam, he is as he said; and whoever says to a believer, 'O disbeliever,' it is like killing him."
Hadith Referenceالمعجم الكبير للطبراني / باب الثاء / 1318