an Imam (the one who leads congregational Prayer) knows during Salah that his wudu' is invalidated, it will be Wajib (obligatory) on him to stop his Salah and choose one of the other worshipers to complete leading the Salah instead of him. However, if the Imam under such condition does not do the foregoing and completes the Salah while he is not in a state of Taharah (ritual purification); both his Salah and that of Ma'mums (people being led by an Imam in Prayer) is considered Batil (null and void) and thus they have to make up for it. Proof for the foregoing is a Hadith which is related by
Muslim and others that the Messenger of Allah (peace be upon him) said:
Allah does not accept prayer without purification.
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.