As it has a free parameter $λ$ Ι have tried to prove it by the WZ method. However it seems that the function $G(n, λ)$ inside the summation is not hypergeometric in its two symbols $n$ and $\lambda$; that is, $G(n+1,\lambda)/G(n,\lambda)$ and $G(n,\lambda+1)/G(n,\lambda)$ are not rational functions.

I have not seen this formula for $\pi$ before in the literature despite that I know many other formulas. Although the series is not good for computing $\pi$ (contrary to what is stated in the press), it could have some interest. In the paper there is also a formula of the same style for $\zeta(2)$.Taking the limit of it as $λ \to \infty$ we obtained a well-known alternating series with a better convergence.