What should a latecomer do if he joins the Imam in the second rak'ah of the prayer, and the Imam forgets and performs an extra rak'ah: should he salute with the Imam or stand up after the Imam's salutation and perform a rak'ah?

The worshipper (ma'mum) offers the salutation of peace (taslim) with the Imam if the Imam has completed his prayer. The Imam's prayer is valid in this addition because he is excused. Whoever is aware of the addition must sit and offer the taslim unless he fears that the Imam stood due to an error. As for the latecomer (masbuq), the additional rak'ah is counted for him, so he offers the taslim with the Imam or performs one rak'ah after the Imam's taslim if he (the Imam) entered the third or fourth rak'ah.