Are there any references for this theorem of Lercher?

Let $$\Delta = \lambda x.(x)x$$ and consider $$\Omega = (\Delta)\Delta$$. Then $$\Omega$$ is exactly the only $$\lambda$$-term of the form $$(\lambda x.t)v$$ such that $$(\lambda x.t)v=t\{v\ /\ x\}$$.

Does someone have references about this statement ? I could not find something enough relevant in my ressources or in my lectures material.