Proposition 7, Book XII of Euclid's Elements
states: Any prism which has a triangular base is divided into three pyramids equal to one another which have triangular bases . ("Equal" means "have the same volume".)
 D. E. Joyce, "Euclid's Elements
 Euclid, Elements
, T. L. Heath, trans., as The Thirteen Books of Euclid’s Elements
, 2nd ed., Book 3, New York: Dover Publications, 1956 p. 394.