Rules reference please (OBR)
 

When there is catcher interference on the batter runner and the BR does not advance to first base. I know that the BR is awarded first base and all other baserunners advance only if forced. Could somewhere tell me the rule reference that says that the runners go back if not forced. Rule 6.08c refers to the option of the manager but it does not specify what to do with the baserunners.

Thanks in advance.

6.08c is right. However, your phraseology of "runners go back" leads me to believe you're not reading it correctly.

Think of the option like football... they take the play, or they take the penalty. Runners aren't "going back" if not forced, per se. If "the penalty" is accepted, the play didn't happen, so there's no "going back". They simply stay where they were before "the play" started (unless forced)..

I'm looking for the part in the rulebook where it says what you're saying in your last sentence?

The play didn't happen. Therefore they can't advance.

OK, now I get it. You let the play finish, but once you impose the penalty the play didn't happen.

If runners were stealing on the play, they get their advance base too.

7.04(d) for rule reference:

7.04 Each runner, other than the batter, may without liability to be put out, advance one base when—
(d) While he is attempting to steal a base, the batter is interfered with by the catcher or any other fielder.