He—may Allah Most High have mercy on him—was asked:

A man committed illicit intercourse with a woman in his youth. Recently, he saw a daughter with her whom he wishes to marry. He does not know whether she is his daughter or another man’s, and is therefore hesitant to proceed. May he marry her?

He replied:

Praise be to Allah. According to most scholars, he may not marry her. Even if she is another man’s daughter by the woman with whom he committed illicit intercourse, marrying her is unlawful according to Abū Hanīfa and Mālik, and according to one of the two reports from Ahmad. If she is his own daughter born of illicit intercourse, the prohibition is even more severe. If he cannot distinguish her from another woman, both are forbidden to him in marriage.

He—may Allah have mercy on him—was also asked:

A man committed illicit intercourse with a woman, who became pregnant by him and gave birth to a girl. May he marry the daughter?

He replied:

Praise be to Allah. This is unlawful according to the overwhelming majority of scholars. None of the Companions or those who followed them in righteousness permitted it. Hence Ahmad ibn Hanbal and other scholars, despite their extensive knowledge, knew of no disagreement concerning this among the early generations. A ruling was therefore given by Ahmad