Musnad Ahmad 31:207

Yahya ibn Sa'id narrated to us, from Hisham, Yahya narrated to us, from Abu Salamah, from Abu Sa'id al-Khudri, from the Prophet (peace and blessings of Allah be upon him), who said: "If you see a funeral procession, then stand for it. Whoever follows it, let him not sit down until it is placed."