Musnad Abu Ya'la al-Mawsili 153:32

Abu Khaythama narrated to us, Yazid bin Harun narrated to us, Nafi' bin 'Umar al-Jumahi narrated to us, from Abu Bakr bin Musa, who said: I was with Salim bin 'Abdullah on a journey, and a caravan belonging to Umm al-Banin passed by, in which there were bells. Salim narrated from his father, from the Prophet (peace and blessings of Allah be upon him), that he said: "The angels do not accompany a caravan in which there is a bell." So how many bells are in this one?