The Muqtadi should not Recite the Thanaa once the Imaam has started the Qiraa'ah

Q: Salaah-733: I joined the Isha salaah after the Imaam had commenced his Qiraa'ah. Should I recite the Thanaa as soon as I join the Jamaa'ah, in Ruku or in Sajdah?

Answer: If the Imaam has started the Qiraa'ah, the Muqtadi must not recite the Thanaa. And Allaah knows best what is most correct.

Fatawa Rahimiyyah vol.2