Musannaf Ibn Abi Shayba 3:3518

Waki’ narrated to us, he said: Hammad bin Zayd narrated to us, from Abu ‘Abd Allah al-Shaqari, from Ibrahim, regarding a man who prayed one rak’ah then broke his wudu (hadath), so he took a man by the hand and brought him forward, and that rak’ah had already passed him (the new imam). He said: He leads them in the remainder of their prayer. When he completes it, he takes a man by the hand from those who witnessed that (first) rak’ah and brings him forward. Then he (the new imam) performs the taslim with them, then he (the original imam) stands and makes up that rak’ah.