indefinite quadratic form in four variables universal over p-adic integers

I need a source for the following statement:

Let $q(x,y)=ax^2+bxy+cy^2$ be a binary quadratic form with $a,b,c\in\mathbb Z$ and let $p$ be a prime with $p\not\mid 2D$, where $D=b^2-4ac$ is not a square. Then the quaternary quadratic form $q(x_1,y_1)-q(x_2,y_2)$ represents all $p$-adic integers, as $x_1,x_2,y_1,y_2$ vary over $\mathbb Z_p$.

I looked for a statement like this in [Cassels, Rational Quadratic Forms] but could'nt find anything. Thank you in advance!