He was sitting with Marwan when a funeral procession passed by. Abu Saeed walked past them and said, "Stand up, commander. This man knows that whenever the Prophet ﷺ followed a funeral procession, he would not sit down until it was placed on the ground."
حدثنا وكيع عن ابن أبي ذئب عن المقبري عن أبيه
أنه كان جالسا مع مروان فمرت جنازة فمر به أبو سعيد فقال قم أيها الأمير فقد علم هذا أن النبي ﷺ كان إذا تبع جنازة لم يجلس حتى توضع
Its isnad is sahih. Bukhari related the hadith, 3/178, no. 1310, in Fath al-Bari, in the chapter on funerals, on one who follows a funeral procession should not sit, and Tirmidhi, 3/331, no. 1020, and Ibn Majah, 1/493, no. 1545, and Ibn Abu Shayba, 3/310.