Musnad Ahmad 31:338

Yahya ibn Adam narrated to us, Zuhayr narrated to us, from Suhayl ibn Abi Salih, from his father, from Abu Sa’id al-Khudri, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When you follow a funeral procession, do not sit down until it is placed [on the ground]."