Musnad Ahmad 1159:22

'Abd al-Rahman narrated to us, from Malik, from Nafi', from Salim ibn 'Abd Allah, from Abu Jarrah, the freed slave of Umm Habibah, from Umm Habibah, from the Prophet (peace and blessings of Allah be upon him), who said: "The caravan that has a bell with it, the angels do not accompany it."