Ruling on the prayer of one who prays a fard prayer behind one who is praying a naafil prayer

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.