There's a quick proof in Yves André's book "Une introduction aux motifs" (proposition 3.4.6.1).
Note that a stronger result is true : actually, algebraic equivalence coincides with numerical equivalence for divisors on $X$ ("Matsusaka's theorem"). The only reference I know for this result is Matsusaka's original article ("The criteria for algebraic equivalence and the torsion group", Amer. J. Math. 79 (1957), 53–66), and I don't know if you would consider it a good reference.