Is it permissible to leave congregational prayer if foul odors are emanating from one of the worshippers? And is there a hadith or a legal text concerning this that could be displayed on the mosque door?

It is not permissible to abstain from congregational prayer due to an unpleasant odor emanating from one of the worshipers. As for the one from whom such odors emanate, it is disliked for him to enter the mosque, and he is permitted to forgo congregational prayer if he cannot remove the odor. This applies whether the odor is due to eating garlic, onions, or similar foods, or if it is a foul smell from the body or mouth, because the underlying reason is the harm caused to the worshipers and the angels. He must remove the odor with purifiers or by using a lot of perfume before going to the mosque.