Field Norm of Quaternion is Multiplicative

Theorem
Let $\mathbf x$ be a quaternion.

Let $\overline {\mathbf x}$ be the conjugate of $\mathbf x$.

The norm of $\mathbf x$:
 * $n \left({\mathbf x}\right) := \left\vert{\mathbf x \overline {\mathbf x} }\right\vert$

is a multiplicative function.