Musnad Ahmad 37:6

Ya'qub ibn Ibrahim narrated to us, 'Abd al-Malik ibn al-Rabi' ibn Sabrah narrated to us, from his father, from his grandfather, who said: The Messenger of Allah, may Allah bless him and grant him peace, said: "The sutrah (barrier) of a man in prayer is an arrow, and if one of you prays, let him use an arrow as a sutrah."