Baseball Experts: Is this a foul ball ?

I was wondering if someone hits the ball and it rolls on the ground towards first base. It crosses the line to first base before the actually base but rolls back inside the line and then hits the base. Is that a foul ball or not? I know if it crosses the line before the base it is foul but this one rolls back inside. Is that a foul? Could you also direct me to a site listing these common rules about baseball strike/foul/ball? Thank you.