It is narrated from Ibrahim (may Allah have mercy on him) that if a man and a woman are caught in the act of adultery and both say that they are husband and wife, then Ibrahim (may Allah have mercy on him) said: "Testimony will be demanded from both of them; if they cannot produce it, the prescribed punishment (hadd) will be carried out on the woman, otherwise every wicked person would escape the hadd."
Hadith Referenceسنن سعید بن منصور / كتاب النكاح / 2191