Musannaf Ibn Abi Shayba 1:34

Ubaydah ibn Humayd narrated to us, from Abd al-Malik ibn Umayr, from Abu Rawh, who said: The Messenger of Allah (may Allah bless him and grant him peace) prayed with his companions and recited Surah al-Rum, but he faltered in it. When he finished, he said: "Our prayer is only confused for us by people who attend the prayer without purification. Whoever attends the prayer, let him perform the purification well."