Musnad Ahmad 199:3

Abu Sa’id, the freed slave of Banu Hashim, narrated to us, Zaidah narrated to us, ‘Abd al-Malik ibn ‘Umayr narrated to us, he said: I heard Shabib Abu Rawh, from Dhu al-Kala’, that he prayed the morning prayer with the Prophet, may God bless him and grant him peace, and he recited [Surah] al-Rum, but he faltered in a verse. When he finished, he said: "The Qur’an is being made confusing for us. Indeed, there are people among you who pray with us but do not perform the ablution well. So, whoever attends the prayer with us, let him perform the ablution well."