بسم الله الرحمن الرحيم
If the imam is not sure whether he performed wudhu or not and it is very much possible that he led salah with wudhu then in such a case salah was valid, there is no need to worry. If after doubt he is fully sure that he led the salah without wudhu then in such a case salah was not valid and it is necessary to reoffer that salah. The imam should announce and inform the musallis about the salah which he led without wudhu so that the musallis reoffer that salah and the imam should also reoffer it with doing taubah and istighfar.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.