Not Sitting in Qa'dah after the Isha salaah and then Performing Six Rakaahs

Q: Salaah-705: After completing four Rakaahs in the Isha salaah, the Imaam went on to complete six Rakaahs without sitting in Qa'dah. Despite the Muqtadis correcting him, he continued and then performed Sajdah Sahw at the end of the sixth Rakaah. Is the salaah valid?

Answer: While the first Qa'dah is Waajib (compulsory) and can be compensated for by Sajdah Sahw, the second Qa'dah is Fardh (obligatory) and Saldah Sahw will not compensate for it. The Fardh salaah was therefore invalid in the stated case and will be regarded as Nafl. The Fardh will have to be repeated. And Allaah knows best what is most correct.

Fatawa Rahimiyyah vol.2