Reading about the geometrical theory of systems of first order pdes, I have met a result from symplectic geometry, that is easy to prove, but I am unable to give a reference for it. So my question is:

Where could I find a reference to the following easy result?

Let $W$ be a coisotropic embedded submanifold of a symplectic manifold $(M,\omega)$. Let $T_W M$ be the restriction to $W$ of the tangent bundle of $M$. In the symplectic vector bundle $(T_W M,\omega|_{T_W M})$, let the symplectic complement of $TW$ be denoted by $(TW)^\perp$.

Then $(TW)^\perp$, being a vector subbundle of $TW$ because of the coisotropy of $W$, is completely integrable.