Performing Four Rakaahs in Error

Q: Salaah-498: There is a Molvi Sahib from Kari who often comes here to Ahmadabad and even leads the salaah sometimes. He once came here and led the salaah as usual, forgetting that he had intended to have further to Bombay and Surat. He was in the final Qa'dah when he realised that he was still a Musaafir, so he made Sajdah Sahw. Will the salaah of the Muqeem followers be valid?

Answer: The salaah of the Imaam and other Musaafir followers will be valid. However, the salaah of the Muqeem followers will be invalid because although the last two Rakaahs were Nafl for the Imaam, they were Fardh for the followers and people performing Fardh salaah cannot follow someone performing Nafl salaah. And Allaah knows best what is most correct.

Fatawa Rahimiyyah vol.2