Musnad Ahmad 311:7

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from Ayyub, from Abu Qilabah, from Thabit ibn al-Dahhak, who attributed the hadith to the Prophet (peace and blessings of Allah be upon him), who said: "Whoever kills himself with something will be punished with it. Whoever testifies against a Muslim—or he said: a believer—with disbelief, it is like killing him. Whoever curses him, it is like killing him. And whoever swears falsely by a religion other than Islam, he is as he swore."