Yosef Karo, Shulchan Aruch — Choshen Mishpat 16
“ When the proof arrives, the defendant can respond. So long as he has not brought the proof, the defendant does not need to respond. If the plaintiff then says I don’t have a proof, the defendant should respond without the plaintiff’s proof. There are those that say that the defendant can swear and will be exempt and the plaintiff can no longer bring a proof. If the defendant requests time to respond to the plaintiff’s claim, if it seems to the court that he just pushing them off, they should not grant the request and the defendant must respond immediately. ”
