Proposition 7, Book XII, Euclid's Elements

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 [2]. ("Equal" means "have the same volume".)
[1] D. E. Joyce, "Euclid's Elements: Introduction."
[2] 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.