If the Imam remembers that he is not in a state of Wudu, so he pulls someone to lead the congregational prayer, and this person does not know which Rak'ah they are in, what should he do?

If the caliph (successor imam) in prayer does not know how many rak'ahs the original imam prayed, Ibn Qudamah holds that he should build upon certainty. If this accords with the truth, then good; otherwise, they (the congregants) should glorify God to alert him. He then returns to their opinion and performs the prostration of forgetfulness (sujud al-sahw). He also mentioned other opinions: such as Al-Nakha'i's saying that he should observe what those behind him do; Al-Shafi'i's saying that he should act according to their response, and if they glorify God, he should sit, knowing it is the fourth rak'ah; Al-Awza'i's saying that he should pray one rak'ah with them, then step back and put forward a man to pray the remainder with them; and Malik's saying that he should pray a complete prayer for himself. The first three opinions are similar, and the preponderant view is that he should build upon certainty due to the absence of conjecture.