Found in Epicurus‘ (341-270 BCE) Letter to Herodotus:
“Moreover, there is an unlimited number of cosmoi, and some are similar to this one and some are dissimilar. For the atoms, which are unlimited…are also carried away to very remote distances. For atoms of the sort from which a world might come to be or by which it might be made are not exhausted [in the production] of one world or any finite number of them, neither worlds like this one nor worlds unlike them. Consequently, there is no obstacle to the unlimitedness of worlds.”
From Brad Inwood and L.P. Gerson, eds., The Epicurus Reader: Selected Writings and Testimonia (Indianapolis, IN: Hackett, 1994), 8.