We want to find the reference book for some version of the Schur's lemma which covers the following result


Let $A$ be an assoiative algebra over $\mathbb{C}$ with countable basis, then any central element acts on any simple $A$-module as a scalar.


Thanks



A reference book for Schur's lemma
This is (an immediate consequence of) [Lemma 2.1.3(b) in Chriss-Ginzburg](http://books.google.com/books?id=lwS59rR78eIC&lpg=PA63&vq=Lemma%202.1.3&pg=PA62#v=snippet&q=Lemma%202.1.3&f=false).


