Musannaf Ibn Abi Shayba 3:4130

Abu Bakr narrated to us, he said: Jarir ibn Abd al-Hamid narrated to us, from Yazid ibn Abi Ziyad, from Abu Fakhita, from Ibn Umar, he said: Tighten your waist-wrapper (izar) in prayer, even if it is with a hobbling rope (iqal).