Do the spouses bear a sin or require expiation if sexual intercourse occurred after the appearance of blood, believing it to be istihadah (non-menstrual vaginal bleeding), and then it became clear that it was menstrual blood that appeared at an unaccustomed time?

The blood seen by the wife after twelve days is not menstrual blood; rather, it is istihada (non-menstrual vaginal bleeding). This is because it is less than the minimum period of purity between two menstrual cycles, which is fifteen days according to the majority of scholars, and thirteen days according to the Hanbalis. Consequently, there is no sin upon the husband for the intercourse that occurred, as intercourse with a woman experiencing istihada is permissible.