If the imam breaks his wudoo’ or remembers that he broke his wudoo’ whilst praying, it is permissible for him to appoint someone to lead the prayer for him
Question

                An imam led the people in prayer, and during the prayer he remembered that he had not done wudoo’ at all. He stopped praying and appointed one of the worshippers to lead the prayer for him. He had prayed one rak’ah, and the person whom he appointed came and continued the prayer from where the imam had left off. Then the imam went and did wudoo’, and repeated the prayer. Is what this imam did correct, knowing that he follows the Shaafa’i madhhab?.
Praise be to Allah.What the 
    imam did, stopping his prayer and appointing one of the worshippers to lead 
    the prayer for him, and what that person did by carrying on the prayer where 
    the imam had left off, was all correct. 
The view 
    that it is permissible to appoint someone else to carry on in such 
    situations is the view of the majority of Hanafi, Maaliki and Shaafa’i 
    scholars, and it was narrated from Imam Ahmad. 
Al-Nawawi 
    (may Allaah have mercy on him) said: Our companions said: If the imam stops 
    praying because of breaking wudoo’ deliberately or otherwise, or that 
    happened before or he forgot or for some other reason, or for no reason, 
    then there are two well known opinions on appointing someone else to carry 
    on leading the prayer. The correct view is that it is permissible because of 
    the saheeh hadeeth. It is proven in al-Saheehayn that the Messenger 
    of Allaah  (peace and blessings of Allaah be upon him) went to reconcile 
    between Banu ‘Amr ibn ‘Awf, and Abu Bakr led the people in prayer. Then the 
    Prophet  (peace and blessings of Allaah be upon him) came during the 
    prayer, and Abu Bakr moved backwards, and the Prophet  (peace and 
    blessings of Allaah be upon him) took over leading the prayer. End quote 
    from al-Majmoo’ (4/138). 
Shaykh Ibn 
    ‘Uthaymeen said in al-Sharh al-Mumti’ (4/243): If the imam remembers 
    during the prayer that he does not have wudoo’, then he has to stop praying 
    and appoint someone to complete the prayer, because when ‘Umar ibn 
    al-Khattab (may Allaah be pleased with him) was stabbed by Abu Lu’lu’ah 
    al-Majoosi, after he had started to pray Subh, ‘Umar took the hand of ‘Abd 
    al-Rahmaan ibn ‘Awf and made him move forward. And he led them in a brief 
    prayer. (Narrated by al-Bukhaari). 
This was in 
    the presence of the Sahaabah, may Allaah be pleased with them. But if he 
    does not do that and he moves away – i.e., without appointing someone to 
    lead the prayer for him – then the members of the congregation have the 
    choice between making one of their number move forward to complete the 
    prayer with them, or completing the prayer individually. End quote. 
And Allaah 
    knows best.