Musnad Ahmad 1018:64

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from ‘Abd al-Malik ibn ‘Umayr, who said: I heard Shabib Abu Rawh narrating from a man among the companions of the Prophet, may Allah bless him and grant him peace, that the Prophet, may Allah bless him and grant him peace, prayed the morning prayer and recited in it: "Ar-Rum" (The Romans), but he became confused in it. He then said: "What prevents me?" Shu‘bah said: He mentioned the armpit (al-rifgh) and the meaning of his statement: "You are not purifying yourselves."