What is the ruling on the prayer of one who prays an obligatory prayer behind one who is praying a supererogatory prayer?

The ruling in this regard is validity, because it has been authentically reported from the Prophet (peace be upon him) that during some of his journeys, he led one group of his companions in the fear prayer (Salat al-Khawf) for two rak'ahs, and then he led another group for two rak'ahs. The second prayer was a supererogatory one for him. Similarly, it is authentically reported in the Sahihayn from Mu'adh (may Allah be pleased with him) that he used to pray the 'Isha' prayer as his obligatory prayer with the Prophet (peace be upon him), and then he would go and lead his community in their obligatory prayer, which was obligatory for them and supererogatory for him. This view is the madhhab (definition) of Imam Al-Shafi'i, and a narration from Imam Ahmad chosen by Sheikh Al-Islam Ibn Taymiyyah, Sheikh Muhammad Ibn Ibrahim, and Sheikh Muhammad Ibn 'Uthaymeen.