How to show that an abelian group is a right $R$-module?

Question

If I want to show that an abelian group $M$ is a right $R$-module, I need to produce a ring homomorphism $\psi: R \to End_{Ab}(M)$.

However, do I need $\psi(rs)=\psi(r) \circ \psi(s)$ or do I need $\psi(rs)=\psi(s) \circ \psi(r)$?