What is the ruling on a husband having intercourse with his wife on the thirty-first day after childbirth, after the bleeding stopped one night earlier, with blood reappearing after intercourse? And does this entail expiation?

If the bleeding of a woman in nifas (postpartum bleeding) stops before forty days and she performs ghusl (ritual bath), it is permissible for her husband to have intercourse with her. If the bleeding returns before forty days, she reverts to the state of nifas, and intercourse with her becomes forbidden until she becomes pure and performs ghusl. If the husband has intercourse with her, assuming her purity, and then the return of bleeding becomes apparent, there is no sin or expiation upon him.