Question Summary:
Is it permissible to offer Sunnāh Salah behind an Imam who is offering his Fard Salāh?

Question Detail:

I offered my Maghrib Salāh behind a person in Oman. After the Salāh was over, one of the people who prayed with us, he was a Musāfir (traveler). He stood up and started making Takbīr for Ishā Salāh. He insisted us to assist him in praying along with him telling us that if we prayed with him, his Ishā Salāh will be offered and our Sunnāh for Maghrib Salāh will also be over (meaning that we won’t have to offer the Sunnāhs of Maghrib Salāh).
I have a doubt that if I prayed along with, it will be considered as Nafl Salāh or it will become a substitute for my Maghrib Salāh.

Answer :

In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.

Sunnāh Salāhs come under the same ruling as Nafl Salāhs. If you offered your Sunnāh Muakkadah  behind an Imām who was offering his Fard Salāh, your Sunnāh Salāh will be valid. [i][ii]
And Allah Ta’āla Knows Best
Mawlana Abdul Hannan Nizami,
Student Darul Iftaa
USA
Checked and Approved by,
Mufti Husain Kadodia.
www.daruliftaa.net