The Messenger of Allah ﷺ said, "When you see a funeral procession, stand up, and whoever follows it should not sit down until it is placed on the ground."
حدثنا يونس بن محمد حدثنا أبان عن يحيى عن أبي سلمة بن عبد الرحمن
أن رسول الله ﷺ قال إذا رأيتم الجنازة فقوموا فمن اتبعها فلا يقعد حتى توضع
Its isnad is sahih. Aban is Ibn Yazid Attar, Yahya is Ibn Saeed Ansari, and Abu Salama is Ibn Abdul-Rahman, and all of them are trustworthy. The hadith has already appeared at 11139.