Musannaf Ibn Abi Shayba 9:876

Khalf ibn Khalifah narrated to us, from Abu Hashim, from Sa’id ibn Jubayr, from Ibn Abbas, regarding a man and a woman, each of whom obtained from the other a limit (i.e., committed illicit intercourse), and he wanted to marry her. He said: "There is no harm; its beginning is fornication (sifah) and its end is marriage (nikah)."