What is the ruling on when a husband was making intercourse to his wife he spotted a little blood and when they finished it was apparent she had she started her period?

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.

If a husband has sexual intercourse with his wife while she is pure and then during intercourse her menses started, it is an obligation on him to withdraw when he realizes that her menses started as it is forbidden to have intercourse with one's wife while in menses. Allaah says (which means): {…so keep away from wives during menstruation.}[Quran 2:222]. If he does not withdraw his sexual organ after knowing that her menses started, then he is sinful and he should seek forgiveness and he should repent. Besides, he should expiate for this by giving one Dinar of gold or half a Dinar of gold in charity. A Dinar weight is 4.25 grams of gold. So he should either give one Dinar [or half a Dinar] of gold or its value in charity. However, if he does not know that blood that came out is menses, or that he did not see the blood until he finished intercourse, then he is not sinful as the Prophet said: "My nation will not be held accountable for things it forgets to do or things that it does by mistake." [At-Tabaraani] 

Allaah Knows best.