The fifth condition: If the place where one wishes to pray is Najis, it should not be so wet that its moisture would reach the body or the dress of the person praying. But, if the place where one places one's forehead while performing Sajdah, is Najis, the prayers will be void, even if that place is dry. And the recommended precaution is that the place where one offers prayers should not be Najis at all.