Musnad Ahmad 1018:11

Waki’ narrated to us, Sufyan narrated to us, from ‘Abd al-Malik ibn ‘Umayr, from Shabib ibn Abi Rawh, from a man among the companions of the Prophet, may Allah bless him and grant him peace, who said: The Messenger of Allah, may Allah bless him and grant him peace, prayed the dawn prayer and recited in them [the two rak'ahs] Surah Ar-Rum, but the recitation became confused for him. When he finished the prayer, he said: "What is the matter with men who attend the prayer with us without purification? Those are the ones who confuse our prayer for us. Whoever attends the prayer with us, let him perform the purification well."