What is the required action from someone who said something about a believer that was not true, being ignorant of the ruling on that, then repented after learning of the noble prophetic hadith: "Whoever says something about a believer that is not true, Allah will make him dwell in the mire of Khabaal until he retracts what he said"?

Praise Allah for knowing what is right and for reviewing the truth, and strive to seek forgiveness for yourself and for the believers. If you repent sincerely from a sin, the sin is lifted from you, and you become like one who has no sin, for the Prophet, peace and blessings be upon him, said: "One who repents from sin is like one who has no sin." As for those whose honor you slandered falsely, it is sufficient for you to seek forgiveness for them and mention them with goodness. If you do not remember them, then general استغفار (seeking forgiveness) for them and for the believers is sufficient for you, because Allah does not burden a soul beyond its capacity.