Musnad Ahmad 28:97

Abu Abd al-Rahman narrated to us, Sa'id narrated to us, Rabi'ah ibn Sayf al-Ma'afiri narrated to me, from Abu Abd al-Rahman al-Hubuli, from Abdullah ibn Amr, that a man asked the Messenger of Allah, may Allah bless him and grant him peace, and said: "O Messenger of Allah, a funeral of a disbeliever passes by us, should we stand for it?" He said: "Yes, stand for it, for you are not standing for it [the disbeliever], you are only standing in reverence to the One who seizes the souls."