See *Nagell, Introduction to number theory*, Theorem 107. Originally it was proved by Dirichlet. 

An extended bibliography on negative Pell’s equation can be found in *Gerasim, I.-Kh.I.
On the genesis of Redei’s theory of the equation x 2 -Dy 2 =-1. (Russian) Zbl 0731.01014
Istor.-Mat. Issled. 32/33, 199-211 (1990)* (available in electronic form).