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A.k.a. the Pigeonhole Principle. Given 
boxes and 
objects, at least one box must contain more than one
object. This statement has important applications in number theory and was first stated by Dirichlet 
in 1834.
See also Fubini Principle
References
Chartrand, G. Introductory Graph Theory. New York: Dover, p. 38, 1985.
Shanks, D. Solved and Unsolved Problems in Number Theory, 4th ed. New York: Chelsea, pp. 161, 1993.