Answered by Shaykh Ilyas Patel
An imam is leading the prayer. Just after finishing his qira’at in the second rak’ah and before ruku’, his wudu breaks. He makes salaam to his left side and then says, “Hasan, finish the prayer” and he steps away to go and do his wudu again. Hasan steps forward and starts leading the prayer from where the imam left off. Is this proper? If not, what is the proper way for the imam to select someone to replace him? Should the imam say salaam when his wudu breaks or just leave immediately? When the original imam in the above situation completes his wudu’ and rejoins the same jama’at in the fourth rak’ah, does he then have to re-pray all the 3 other rak’at again or just the 2 that he missed (i.e. one in which his wudu’ broke and one which Hasan led while he was making wudu’?
The sunnah procedure to lead the way for a successor (khalifa) is that, the Imam should bend his back and hold his nose, in order to show and make others think, that he has had nose-bleeding, then make a gesture or hold the clothes of a person from the first row, and drag him towards the qibla. The Imam should not speak to this person. If the Imam has him a successor (khalifa), by speaking to him, whether intentionally, by mistake or out of ignorance, the salat of everyone will be void and nullified.
(Allama Zuwwar Husain Shah, Umdatu’l-Fiqh pg 234 V.2)
In the above case, because the Imam spoke to Hasan in order to make him a successor (Khalifa) the salat will be void.