See *Nagell, Introduction to number theory*, Theorem 107. Originally it was proved by Legendre, see *Dickson, L. E. History of the theory of numbers. Vol. II*, p. 365. There are more examples due to Dirichlet, see *Dirichlet's Werke, Bd. 1 (1889)*, pp. 219-236.

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 $x^2 -Dy^2 =-1$.][1] (Russian) Zbl 0731.01014
Istor.-Mat. Issled. 32/33, 199-211 (1990)*.


  [1]: https://pyrkov-professor.ru/Portals/0/Mediateka/IMI/istorikomatematicheskie_issledovaniya_1990_no32-33.djvu