To what extent is the prayer of Mu'adh legitimate, as he used to pray the obligatory prayer with the Messenger of Allah, may Allah's peace and blessings be upon him, and then lead his people in a supererogatory prayer while they were performing the obligatory prayer?

The objection regarding Mu'adh's hadith is sound, and it is the relied-upon evidence for the permissibility of a person performing a fard prayer behind someone performing a nafl prayer, and for praying Dhuhr behind someone praying Asr. Those who prohibit this relied on other evidence and responded to Mu'adh's hadith by saying that he would perform a nafl prayer and then lead his people in a fard prayer. Ash-Shafi'i and others refuted this response. An-Nawawi clarified that the Shaf'i school of thought permits a person performing a nafl or fard prayer to pray behind someone performing a nafl or fard prayer in a different fard prayer, and he cited a number of scholars who held this view. There are those who prohibited this, and among them are those who only permitted a nafl prayer behind a fard prayer. The prohibitors cited the hadith, "Indeed, the Imam was appointed to be followed," while those who permitted it cited Mu'adh's hadith.