The Zassenhaus formula is
$e^{t(X+Y)}= e^{tX}~ e^{tY} ~e^{-\frac{t^2}{2} [X,Y]} ~ e^{\frac{t^3}{6}(2[Y,[X,Y]]+ [X,[X,Y]] )} ~ e^{\frac{-t^4}{24}([[[X,Y],X],X] + 3[[[X,Y],X],Y] + 3[[[X,Y],Y],Y]) } \cdots$
from this Wikipedia page.
$X$ and $Y$ are linear operators, and $[X,Y]$ is their commutator.
I mostly want to prove it for the case where the commutator of $X$ and $Y$ is a constant, or simply the general proof.
What I'm looking for is either a reference to a place where it's proven or at least some shove in the right direction.
So far I've tried to just expand the exponential to see if I see anything but have no ideas so far.