# complex reductive Lie group

I am reading A. L. Oniscik's paper Decompositions of Reductive Lie Groups, and the author cited a proposition that a complex reductive Lie group $G=ZS$ is locally isomorphic to the reductive algebraic group $\widetilde G= \mathbb C^{m}\times S$. I want to have a reference about this proposition. Any reference will be helpful. Thanks.