Continuous Maps, Homomorphisms and Cyclic Groups

Let f: S^n --> S^n be a continuous map.
Consider the induced homomorphism f*: H~_n (S^n) --> H~_n (S^n), where
H~_n is a reduced homology group. Then from the fact that H~_n (S^n) is an infinite
cyclic group, it follows that there is a unique integer d such that
f*(u) = du for any u in H~_n (S^n).

How exactly does "there is a unique integer d such that..." follow from
"H~_n (S^n) is an infinite cyclic group"?