Derivative Function on Set of Functions induces Equivalence Relation

Theorem
Let $X$ be the set of real functions $f: \R \to \R$ which possess continuous derivatives.

Let $\mathcal R \subseteq X \times X$ be the relation on $X$ defined as:
 * $\mathcal R = \left\{{\left({f, g}\right) \in X \times X: D f = D g}\right\}$

where $D f$ denotes the first derivative of $f$.

Then $\mathcal R$ is an equivalence relation.