What is the Ruling on Fasting on Eid Believing it is a Day of Ramadan?

If this fasting is from his Ijtihad and knowledge and not otherwise, there is no sin upon him because he is a Mujtahid (striving to find the correct ruling). So, he is rewarded in every situation. However, the person who is prohibited from fasting is the one who knows it is the day of Eid, and despite that, he fasts it. This is the one for whom the known prohibition is applied.