Shaykh ‘Abd al-‘Azeez ibn Baaz said: “The ruling is that that is valid, because it was proven that on some of his journeys, the Prophet (peace and blessings of Allaah be upon him) led one group of his companions in praying two rak’ahs of the fear prayer, then he led another group in praying two rak’ahs, so the second prayer was naafil for him. Similarly it was proven in al-Saheehayn from Mu’aadh (may Allaah be pleased with him) that he used to pray the fard of ‘Isha’ with the Prophet (peace and blessings of Allaah be upon him), then he would go and lead his own people in praying their fard prayer, so it would be fard for them and naafil for him. (See Saheeh al-Bukhaari, 701; Saheeh Muslim, 465).”                                                                             

(Majmoo’ Fataawa al-Shaykh ‘Abd al-‘Azeez ibn Baaz, 12/178)                               

This opinion is the view of Imaam al-Shaafa’i and was narrated in one report from Imaam Ahmad. It was also the view favoured by Shaykh al-Islam Ibn Taymiyah, Shaykh Muhammad ibn Ibraaheem and Shaykh Muhammad ibn ‘Uthaymeen (may Allaah have mercy on them all). See al-Sharh al-Mumti’, 4/358.