Sayyiduna Ibn Abbas (may Allah be pleased with them both) narrates that a man came and said: O Messenger of Allah, I had relations with my wife while she was menstruating. The Prophet (peace and blessings be upon him) ordered him to free a slave, and the price of a slave at that time was one dinar.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12204