What is the ruling regarding a person who disbelieved (kafara), then led the people in prayer (amma), and we do not know whether he repented or not?

Among the conditions for the validity of following an Imam is that he must be a Muslim. Thus, it is not permissible to follow a disbeliever (Kafir), whether he is an original disbeliever or an apostate (Murtad). If an apostate leads people in prayer, the prayer of those behind him is invalid unless they know of his repentance before the prayer. Imam Al-Shafi'i said: "If a Muslim man apostatized, then led prayer while he was an apostate, the prayer of those behind him would not be valid until he outwardly declared repentance with words before their imamate." If they do not know his status, it is recommended for them to repeat the prayer.