Musnad Ahmad 30:475

Muhammad ibn Salamah narrated to us, from Ibn Ishaq, from Muhammad ibn Ibrahim, who said: I came to Sa'id ibn Marjanah and asked him, and he said: I heard Abu Hurairah saying: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever prays over a funeral and does not walk with it, let him stand until it disappears from his sight. And whoever walks with it, let him not sit down until it is placed [on the ground]."