Center of Division Ring is Subfield

Theorem
Let $\struct {K, +, \circ}$ be an division ring.

Then $\map Z K$, the center of $K$, is a subfield of $K$.

Proof
For $\map Z K$ to be a subfield of $K$, it needs to be a division ring that is commutative.

Thus the result follows directly from Center of Ring is Commutative Subring.