What must a person do who caught up with the Imam after he rose from the first rak'ah and then, out of ignorance that he had missed a rak'ah, said the taslim with the Imam?

If this person performed their prayer incompletely, even if they were unaware of it, and the ruling was clarified to them after they had concluded the prayer but before a long interval had passed, then they must perform one rak'ah without an opening takbir (takbirat al-ihram). Then they should perform the prostration of forgetfulness (sujud al-sahw) before the salutation (taslim) according to the Shafi'i school, or after it according to other schools. However, if a long interval has passed, or they have spoken much, or moved excessively, then they must repeat this prayer.