I've done it below, check it.
From Newton's second law, we know
Now, by definition of momentum,
We thus have,
For a constant mass
By definition of acceleration,
We therefore have the result,
Now, for a zero external force,
For a non zero mass, we have
In other words, when no (net) external force acts on the body, if it is in rest, it continues to be in it's state of rest or if it is moving along a straight line, it continues to move along it neither gaining nor losing speed.
This is the Newton's first law.