Musnad Abu Ya'la al-Mawsili 238:8

Ishaq narrated to us, Sufyan narrated to us, from al-Zuhri, from Salim, from his father, from Amir ibn Rabi'ah, that the Prophet (peace and blessings of Allah be upon him) said: "If you see a funeral procession, then stand up for it until it passes behind you or it is placed down."