A person joins the Imaam in the final sitting posture of Salaat. After joining the Imaam, the Imaam makes Salaam. The Muqtadi has not yet completed the Tashahud. Is it necessary for him to complete the Tashahud? If he omitted the Tashahud, is Sajdah-Sahw necessary at the end of the Salaat? 

Answer: 

If a person joined the Imaam in the final sitting posture, and did not complete the Tashahud when the Imaam makes Salaam, then this person should not stand up immediately, rather, he should complete the Tashahud and then stand up. if he did not recite the Tashahud, the Salaat will be in order. (Raddul Muhtaar Vol. 3 Pg. 314)