What is the ruling on one who has sexual intercourse with his deceased wife out of love and farewell, knowing that she is dead?

Having sexual intercourse with a deceased person is a prohibited act that a sound mind and a pure natural disposition would abhor. There is no explicit religious text mandating a prescribed punishment (حد) for the perpetrator, but he should be disciplined.

Three consequences result for the one who commits this prohibited act: performing ghusl (ritual bath) for major impurity, although re-washing the deceased is not required, and he must repent to Allah Almighty and frequently seek forgiveness.