Sahih Muslim Book 1, Hadith Number 203.

Chapter : Suicide is the gravest sin.

It is narrated on the authority of Thabit b. Dahhak that the Apostle of Allah (may peace be upon him) observed: He who took deliberately a false oath on a religion other than Islam would become that which he had professed. And he who killed himself with anything Allah would torment him with that in the Fire of Hell.