How should an imam act if his ablution is invalidated during prayer?

If an imam's ablution is invalidated during prayer due to an involuntary reason, such as a nosebleed, he should appoint a suitable person from the congregation behind him to take his place at the mihrab. This is called "istihlaf." (Merghinani, al-Hidaya, 1/59) For example, when Umar (r.a.) was attacked and wounded while leading the prayer, he appointed Abdurrahman b. Awf (r.a.) to continue as imam. Similarly, when Ali (r.a.) experienced a nosebleed while leading the congregation, he appointed someone from the congregation to take his place. (Aliyy al-Qari, Fath bab al-‘inaya, 1/346) If the imam whose ablution is invalidated leaves the imamate without appointing someone in his place, the congregation’s prayer becomes invalid. (Kasani, Bada’i, 1/226)

Source: T.C. Diyanet İşleri Başkanlığı, Din İşleri Yüksek Kurulu