elementary number theory - Prove that ${ ax+bymid x,yinmathbb Z} = { n(a,b) mid ninmathbb Z}$

Prove the following proposition:

Suppose $a,b$ are fixed integers. Then $\{ ax+by\mid x,y\in\mathbb Z\} = \{ n(a,b) \mid n\in\mathbb Z\}$.