Linear Operator $A$ and $B$ bounded if $(Ax,y) = (x,By)$?

Question

Suppose $A,B$ are linear operators on Hilbert Space $\mathcal{H}$, satisfy $$ (Ax,y) = (x,By) $$ for $\forall x,y \in \mathcal{H}$. Show that $A,B$ are bounded.

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What I can get so far : $A,B$ are almost 'adjoint', except that we don't know yet if they are bounded.

Thanks for helping.