What should the Imam do if his ablution is nullified while he is prostrating, and is it legislated for him to say "Allahu Akbar" so that the congregants may stand, then appoint someone else to lead?

If the Imam's ablution (wudu') is nullified while he is prostrating, he should raise his head without saying "Allahu Akbar" (God is Greatest), and instruct someone behind him to lead the congregational prayer. The new Imam then says "Allahu Akbar" to rise from prostration and completes the prayer. If the first Imam mistakenly says "Allahu Akbar," and the congregants raise their heads upon hearing his takbir, their prayer is not invalidated.