Question: We hope from you a clarification on how the late comer to prayer should sit if he finds the Imam in the last rak’ah? Does he supplicate [as if he is in] the first or last Tashahhud?
Answer: “If the latecomer comes and the Imam is in the last Tashahhud, then he sits like how the Imam sits and how the congregants who are not latecomers. He sits mutawarrik as comes in the Sunnah of the Messenger of Allah (ﷺ). So the Imam sits in mutawarrik form, and those behind him, and the latecomer. He should not consider himself to be in the first Tashahhud. Rather he performs the Tashahhud, sends salah on the Prophet (ﷺ), supplicates, and increases in supplication until the Imam makes Tasleem.
So he follows the Imam in the manner of sitting, and in Tashahhud, and sending salutations upon the Prophet (ﷺ). And he chooses what he wills from supplication as come from the Messenger of Allah (ﷺ). That does not mean he sits in Tashahhud and is silent. Rather he does what the Imam does”.
[Sharh Sunan Abi Dawood no. 128]
Faisal bin Abdul Qaadir bin Hassan