The ruling on one who prayed the obligatory prayer behind one who was praying a supererogatory prayer
What is the ruling on the one who prays an obligatory prayer behind one who is praying a supererogatory prayer?
The ruling on this is that it is correct because it has been authentically reported from the Prophet Sallallahu alaihi wasallam that while he was travelling, he led a number of the Companions, may Allah be pleased with them, in Salatul-Khawf (the fear prayer), praying two Rak'ahs, then he prayed two more Rak'ahs, so the second prayer was a supererogatory prayer for him. And it is also authentically reported in the Two Sahihs, from Mu'ath, may Allah be pleased with him, that he used to pray the Isha' prayer with the Prophet Sallallahu alaihi wasallam as a obligatory prayer, then he would lead his people in their obligatory prayer which was a supererogatory prayer for him. And Allah is the Granter of success.
Shaykh `Abdul-`Azeez Bin Baz
Fatawa Islamiyah Darussalam Vol:2 page no.139