Solution to Abstract Algebra by Dummit & Foote 3rd edition Chapter 1.1 Exercise 1.1.33
Let $G$ be a group, and let $x \in G$ be an element of infinite order. Prove that the elements $x^k$ with $k \in \mathbb{Z}$ are all distinct.
Solution: Suppose to the contrary that $x^a = x^b$ for some (distinct) integers $a$ and $b$; without loss of generality say $a < b$. Then we have $x^{b-a} = 1$ and $0 < b-a$. This is a contradiction since $x$ has infinite order; thus no such $a$ and $b$ exist.
