Musnad Ahmad 788:7

'Abd al-Samad narrated to us, he said: My father narrated to us, he said: Layth narrated to us, from Abu Burdah, from Abu Musa, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "If a funeral of a Jew, a Christian, or a Muslim passes by you, then stand up for it. For you are not standing up for it; you are only standing up for those who are with it from among the angels."