This famous quote is from the Eleventh Chapter of the Bhagavad-gita, entitled Visva-Rupa-Darsana-yoga, or the “Yoga of Theophany”, the chapter in which Krishna displays His Universal form–His divine Opulence–to Arjuna.
In full, the verse is as follows:
The Lord said: “Time [death] I am, the destroyer of the worlds, who has come to annihilate everyone. Even without your taking part all those arrayed in the [two] opposing ranks will be slain!”
If a thousand suns were to rise together one morning, that light would be a little like the glory of the Lord, for I am become Death, the shatterer of worlds.
Oppenheimer, quoting the Bhagavad Gita upon witnessing the first artificial atomic detonation
It's a little-known fact that Oppenheimer's first love in physics was not particle theory, as such, but gravitation, and published the first papers that explained the theoretical bases of neutron stars and black holes. I've often wondered what would have happened if he'd stopped smoking cigarettes (the cause of the cancer that killed him) and held on for a few more years. He and Stephen Hawking (or Carl Sagan) would have made an interesting pair, and his artistic/philosophical leanings and notoriously fine-tuned sensorium (he was one of the first notable Americans to have a passion for obscure Asian food, among other things) would have been right at home in the Seventies.