Musnad Abu Ya'la al-Mawsili 42:184

Abu Khaythama narrated to us, Abu ‘Amir narrated to us, Hisham narrated to us, from Yahya, from Abu Salamah, from Abu Sa’id, that the Prophet (peace and blessings of Allah be upon him) said: "If you see the funeral procession, then stand up. And whoever follows it, let him not sit down until it is placed [on the ground]."