Prove if $a|b$ and $b|a$, then $|a|=|b|$ , $a, b$ are integers.

Prove if $a|b$ and $b|a$, then $|a|=|b|$ , $a, b$ are integers.

Form the assumption, we can say $b=ak$ ,$k$ integer, $a=bm$, $m$ integer.
Intuitively, this conjecture makes sense.But I can't make further step.