Musnad Ahmad 199:1

Ishaq ibn Yusuf narrated to us, from Sharik, from Abd al-Malik ibn Umayr, from Abu Ruh al-Kala'i, who said: The Messenger of Allah (peace and blessings of Allah be upon him) led us in a prayer, and he recited Surah al-Rum in it, but he became confused in some of it. He said: "The devil only caused confusion for us in the recitation because of people who come to the prayer without wudu (ablution). So when you come to the prayer, perform wudu well."