He attributed it to the Prophet ﷺ, "When you see a funeral procession, stand up for it until it passes you or is set down."
حدثنا مسدد حدثنا سفيان عن الزهري عن سالم عن أبيه
يبلغ به النبي ﷺ إذا رأيتم الجنازة فقوموا لها حتى تخلفكم أو توضع
Bukhari narrated it in the chapters on funerals, in the section on standing for the funeral, hadith no. 1307, and Muslim narrated it in the chapters on funerals, in the section on standing for the funeral, hadith no. 958, from the hadith of Sufyan bin Uyayna with this same chain.