If the Imam dies while I am praying behind him, what should I do?

If an Imam experiences an excuse that prevents him from completing the prayer, the prayer of the congregants is not invalidated; rather, it remains valid according to the majority of scholars. The congregants can choose someone among them to complete the prayer with them. The Malikis hold that appointing a successor is obligatory for Friday prayer and optional for other prayers. The evidence for the majority is the story of Umar appointing Abd al-Rahman ibn Awf as his successor after he was stabbed.