Making Salaam after Three Rakaahs of a Four Rakaah Salaah and then Talking

Q: Salaah-735: The Imaam performed only three Rakaahs for Zuhr and then made the Salaam. When the Muqtadis informed him of this and advised that the salaah be repeated, the Imaam said that it would suffice to merely perform the additional Rakaah. He then proceeded to perform the additional Rakaah, performed Sajdah Sahw and then concluded the salaah. Is this correct? Furthermore, someone joined the Jamaa'ah only in this additional Rakaah. Will his salaah be valid?

Answer: When the Imaam started to speak after completing the third Rakaah, he thereby exited from his salaah and the three Rakaahs were thus nullified. The salaah was therefore not completed by adding the fourth Rakaah and performing Sajdah Sahw. The entire four Rakaahs will now have to repeated. The salaah of the person joining in the additional Rakaah will not be made. And Allaah knows best what is most correct.

Fatawa Rahimiyyah vol.2