It is narrated from Sayyiduna Thabit bin al-Dahhak (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "There is no vow upon a man in what he 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 accuses a believer of disbelief, it is like killing him."
Hadith Referenceالمعجم الكبير للطبراني / باب الثاء / 1319