Musannaf Ibn Abi Shayba 28:971

Abu Usama narrated to us, he said: Abd al-Rahman ibn Yazid ibn Jabir narrated to us, he said: Al-Qasim narrated to me, from Abu Umamah, he said: "Put necklaces on them, and do not put strings (bowstrings) on them," meaning the horses, "and do not put strings on them."