Offering obligatory Prayer behind an Imam offering a Nafilah Prayer

A: It is permissible according to the most correct opinions of the scholars for whoever enters the Masjid after the congregation is over and finds a man offering Sunnah (supererogatory) Salah to join him with the intention of offering the obligatory Salah. This is because Mu`adh (may Allah be pleased with him) used to offer Salah with the Prophet (peace be upon him) then he used to go to lead his people in Salah which is the obligatory Salah for them, whereas it is Nafilah (supererogatory) on his part. The Prophet (peace be upon him) allowed him to do so.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.