I'm looking for the original reference where it was proved that given a subset $X$ of a space $E$ and a function $f:E \times E \rightarrow \mathbb{R}$, the equilibrium problem of finding $x \in X$ such that $f(x,y) \geq 0$ for all $y \in X$ is equivalent to finding the zero of the monotone operator
$$ T = \partial f(x,\cdot) + N_X(\cdot)$$ where $N_X$ denotes the tangent cone of $X$.
Anyone knows who proved this result first?