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: "Whoever kills himself with poison will be punished with it; and whoever bears witness against a Muslim, or he said: against a believer, of disbelief, it is like killing him; and whoever curses him, it is like killing him; and whoever swears falsely by a religion other than Islam, he is as he said."
Hadith Referenceالمعجم الكبير للطبراني / باب الثاء / 1311