The New Musalli who Joined s Salaah that was Being Repeated to, not Performing Sajdah Sahw

Q: Salaah-8: The lmaam forgetfully omitted a Waajib act in salaah and also forgot to perform the Sajdah Sahw. The salaah therefore needed to be repeated. Now will the salaah be valid of a person who joined the Jamaa'ah only when the repeated salaah was being performed?

Answer: His salaah will be invalid and will have to be repeated because the second salaah of the Imaam's is not a Fardh salaah in its own right but a completion of another Fardh salaah, It is not correct for a person performing a fully-fledged Fardh salaah to follow an Imaam who is merely completing an incomplete Fardh salaah.
And Allaah knows best what is most correct.

Fatawa Rahimiyyah vol.2