Musnad Ahmad 3:28

Abdullah narrated to us, Isma'il Abu Ma'mar narrated to us, Yahya bin Salim al-Ta'ifi narrated to us, from Isma'il bin Umayyah, from Musa bin 'Imran bin Manah, from Aban bin 'Uthman, From 'Uthman: That he saw a funeral procession and stood up for it, and he said: I saw the Messenger of Allah, may Allah bless him and grant him peace, see a funeral procession and he stood up for it.