the answer is on page 189 of <A HREF="https://books.google.nl/books?id=VvBAvf0wSrIC&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false">Field Quantization</A> by Greiner & Reinhardt (their $\zeta$ is your $\lambda$):

<IMG SRC="http://ilorentz.org/beenakker/MO/commutator.png"/>