Is my statement, "and therefore it is forbidden to fast on the day of Eid," considered a slander against the Messenger of Allah and speaking without knowledge, and what are the conditions for repentance if that is the case?

It was previously explained that the ruling on fasting on 'Arafah if one's country's sighting contradicts the sighting of the people of Makkah, and that the difference in moonrise is Islamically recognized. Therefore, what you have been informed of has a valid basis, and you will not incur sin by it, nor are you required to inform those you previously informed about the existence of a difference of opinion on the matter.