Gaia Told Him

But when Zeus was full-grown, he took Metis, daughter of Ocean, to help him, and she gave Cronus a drug to swallow, which forced him to disgorge first the stone and then the children whom he had swallowed, and with their aid Zeus waged the war against Cronus and the Titans. They fought for ten years, and Earth prophesied victory to Zeus if he should have as allies those who had been hurled down to Tartarus. So he slew their jailoress Campe, and loosed their bonds. And the Cyclopes then gave Zeus thunder and lightning and a thunderbolt, and on Pluto they bestowed a helmet and on Poseidon a trident. Armed with these weapons the gods overcame the Titans, shut them up in Tartarus, and appointed the Hundred-handers their guards; but they themselves cast lots for the sovereignty, and to Zeus was allotted the dominion of the sky, to Poseidon the dominion of the sea, and to Pluto the dominion in Hades.


- Pseudo-Apollodorus, The Library, Book 1, Chapter 2, Section 1