Should I follow the Imam in the pre-emptive prostration (sujud al-qabli) after catching the third rak'ah of the Asr prayer, while the Imam erred by adding an extra prostration, or should I complete the two rak'ahs and then perform the prostration of forgetfulness (sujud al-sahw)?

If the Imam makes a mistake in his prayer and performs the prostration of forgetfulness (sujud (definition) al-sahw), the congregant (ma'mum) is obligated to follow him in that, whether the congregant also made a mistake with him or the Imam alone made the mistake. Even if the congregant joined the prayer late (masbuq) and did not witness the Imam's mistake, he must still follow. If the congregant intentionally fails to follow the Imam, his prayer becomes invalid. The congregant prostrates with the Imam for the prostration of forgetfulness, then, after the Imam gives the salutations (taslim (definition)), the congregant stands to make up what he missed.