A man married a woman and she had children with him, then he discovered after a period of time that she married his father before. What should he do and what is the ruling about the children he had with her?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

He should have separated from her as soon as he knew the fact because she is eternally forbidden for him since she was his father's wife. Allaah Says (what means): {And do not marry those [women] whom your fathers married}[Quran 4:22]. As regards the children, they are legitimate children and they should be traced back to their father because when the marriage contract was conducted, it was correct, and it only became invalid at the time he knew that she was his father's wife.

Allaah Knows best.