Mukhtasar Bukhari (Abridged) 2031 — Prohibition of cursing

Narrated from Sayyiduna Thabit bin Dhahak (may Allah be pleased with him) that the Noble Prophet (peace and blessings of Allah be upon him) said: "Whoever swears by any religion other than Islam (i.e., deliberately swears to become a disbeliever), then he is as he has sworn (i.e., of that religion). And it is not obligatory upon the son of Adam to fulfill a vow which is beyond his capacity. And whoever commits suicide with anything in this world, he will be punished with that very thing on the Day of Resurrection. And whoever curses a believer, (its sin) is equal to killing him. And whoever calls a believer a disbeliever, this too is equal to killing him."