Musannaf Ibn Abi Shayba 28:970

Waki’ narrated to us, he said: Ibn ‘Awn narrated to us, from Sa’id al-Bazzar, from Makhul, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Put necklaces (qala'id) on them, and do not put necklaces of bowstrings (awtar) on them," meaning horses.