I'm looking for some advice please.

Two of my colleagues have been falsifying their timesheets at work and it is starting to bug me. I only notice because I am observant and not because I am nosey.

Basically it works by you enter the time that you enter each day and the time that you leave.

For example, they are adding around 15 minutes extra to their timesheets making it look like they've worked longer than they actually have.
So say they enter at 12pm and leave at 4pm, they'll note on the sheet that they left at 4:15pm.

I know for a fact that that this is going on and to be honest I'm finding it harder to 'bite my tongue' on this subject.

So, plainly put, would you report them to HR or just let it go?

I'm an honest person and this amount to gross misconduct/theft in my eyes.