What do the followers do if the imam's prayer is invalid?

Q: The questioner, A. M. A., from Aswan in the Arab Republic of Egypt, says: After the imam had prayed two rak'ahs of the 'Isha' prayer, he remembered that he was not in a state of wudu'. He broke off his prayer and turned to the followers behind him to have one of them step forward to lead the prayer in his place. He found them to be common people who did not know how to recite properly, and the one who knew how to recite was somewhat far from him. How should this person step forward to lead the prayer in his place? Should he break off his prayer and then say the takbir again to lead the followers, or should he walk until he reaches the imam's position while still in his prayer? And what if the person asked to step forward is in the second row? Please give us a ruling, and may Allah reward you.

A: It is preferable for the imam to appoint a substitute to complete the prayer if something occurs that necessitates his leaving the prayer. If that is not feasible, each person should complete the prayer for himself. If they wait until he returns to pray with them, there is no harm in that. 'Umar (may Allah be pleased with him) appointed a substitute when he was stabbed while in prayer; he appointed 'Abd al-Rahman ibn 'Awf (may Allah be pleased with him), who completed the prayer for the people. If the substitute is not behind the imam and takes a few steps to the position of the imam, there is no harm in that; for it is authentically reported from the Prophet (peace and blessings be upon him) what indicates this, when he prayed with the people on the minbar, then descended and prostrated on the ground to teach them the prayer in such a case, so that they would follow him. Likewise, in the prayer for the eclipse, when Paradise was shown to him, he stepped forward, and the people stepped forward with him. And when Hellfire was shown to him, he stepped back, and they stepped back. And success is from Allah.