Is the Imam's ablution nullified in congregational, paired, or individual prayer, and does the congregants' prayer become invalid as a result?

If the Imam's ablution (wudu') is nullified during prayer, it is sunnah for him to appoint one of the worshippers to complete the prayer with them. His own prayer becomes invalid due to his impurity, whether he was leading a congregation or praying alone, and he must discontinue his prayer and perform ablution to pray anew. The prayer of the congregants is not invalidated by the Imam's impurity if they do not continue to follow him after his impurity and they are aware of the invalidity of his prayer.