You can add a quiet 'no', nothing wrong with that. The point is not to punish the dog or in any way show that you are mad. You have to interrupt and take them directly outside. Remember, you are teaching the dog 'where to go' and not only 'where not to go'. He needs to associate being outside with potty breaks. Thats why he should not be allowed to roam free around the house withour supervision. If he is caught in the act, he should be immediately taken outside.