How do the followers complete their prayer if the imam's prayer is invalid

Q: A questioner asks and says: The imam left the prayer and did not appoint a replacement. Should I step forward to lead the people? Or should I complete the prayer on my own?

A: It is prescribed for the imam, if he breaks his ritual purity during the prayer, or if he remembers that he entered the prayer without being in a state of ritual purity, to turn away and appoint someone to complete the prayer for the people, as 'Umar (may Allah be pleased with him) didwhen he was stabbed during the Fajr prayer; he appointed 'Abd al-Rahman ibn 'Awf, who completed the prayer for the people. If he does not appoint someone, it is prescribed for those behind him that one of them should step forward and complete the prayer for the people. If they do not do so, each person should complete the prayer for himself, because he is excused, as the scholars have explained. And success is from Allah.