Musnad Ahmad 31:461

'Abd al-Malik ibn 'Amr narrated to us, Hisham narrated to us, from Yahya, from Abu Salamah, from Abu Sa'id al-Khudri, that the Prophet (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]."