A Musaafir Performed salaah behind a Muqeem Imam and later learnt that the salaah was invalid

Q: Salaah-504: A Musaafir performed salaah behind a Muqeem Imaam and only later learnt that the salaah was invalid. When repeating the salaah, should he perform Qasr or not?

Answer: When the salaah was nullified in a manner that even Sajdah Sahw cannot mend (such as missing out a posture), the Musaafir will repeat the salaah in full because the obligation of salaah was not discharged at all. If the salaah is repeated with the Imaam, it will be performed as four Rakaahs. However, if the error was such that Sajdah Sahw would have rectified it, the Musaafir should repeat the salaah as four Rakaahs since the obligation of salaah was discharged with the Imaam and the repetition is for completion. And Allaah knows best what is most correct.

Fatawa Rahimiyyah vol.2