Musnad Ahmad 31:376

Yunus ibn Muhammad narrated to us, Aban narrated to us, from Yahya, from Abu Salamah ibn Abd al-Rahman, from Abu Sa’id al-Khudri, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "When you see the funeral procession, stand up. Whoever follows it, let him not sit down until it is placed [on the ground]."