Musnad Ahmad 311:1

Yahya ibn Sa'id narrated to us, he said: Hisham and Yazid narrated to us, they said: Hisham informed us, he said: Yahya narrated to me, from Abu Qilabah, from Thabit ibn al-Dahhak, that the Prophet, peace and blessings of Allah be upon him, said: "Cursing a believer is like killing him, and whoever kills himself with something in this world will be punished with it in the Hereafter. There is no vow upon a Muslim man regarding that which he does not possess. Whoever accuses a believer of disbelief, it is as if he has killed him. And whoever swears by a religion other than Islam, while lying, then he is as he said."