If you were inattentive during prayer behind the Imam, then realized that you had not missed a rak'ah, is it permissible for you to sit and say the Tasleem immediately, or is Sujood As-Sahw (prostration of forgetfulness) obligatory upon you?

It is known that the Imam bears the forgetfulness (sahw) of the follower (ma'mum) as long as he is following him. However, if the Imam makes the taslim (pronounces the salutation of peace), the following ceases. If the follower forgets after his Imam has made the taslim, he should perform the prostration of forgetfulness (sujud al-sahw), whether he was a latecomer (masbuq) or not. The prostration of forgetfulness in this case is obligatory according to the Hanbalis, and sunnah according to the Shafi'is and Malikis.