I need a reference for the result which gives the number of solutions to the congruence $mx^2+ny^2 \equiv k\pmod{p}$. This result seems to be something that would be discussed in Gauss' Disquisitiones Arithmeticae, as it is proven from basic results in the theory of curves over finite fields.

Is anyone aware of a specific reference where the number of solutions to the above congruence is discussed?