Musannaf Ibn Abi Shayba 3:2195

Hushaym narrated to us, from Yunus, from al-Hasan, that he was asked about a man who led a people in prayer during the month of Ramadan while he was without ablution (wudu), and he prayed with them the Isha prayer, the prayer of Ramadan (Tarawih), and the Witr. He said: "He repeats [the prayer], but those behind him do not repeat."