Sunan an-Nasa'i, Volume 3, Book 21, Hadith 1915 (DarusSalam) ⓘ
It was narrated from Abu Sa'eed that the Messenger of Allah said: "When a funeral passes by you, stand up, and whoever follows it, let him not sit down until it is put down (in the grave)."
أَخْبَرَنَا يَحْيَى بْنُ دُرُسْتَ، قَالَ حَدَّثَنَا أَبُو إِسْمَاعِيلَ، عَنْ يَحْيَى، أَنَّ أَبَا سَلَمَةَ، حَدَّثَهُ عَنْ أَبِي سَعِيدٍ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " إِذَا مَرَّتْ بِكُمْ جَنَازَةٌ فَقُومُوا فَمَنْ تَبِعَهَا فَلاَ يَقْعُدْ حَتَّى تُوضَعَ " .
also referenced under: Hadith 1914 (Reference) & Book 21, Hadith 97 (In-Book)