ENOfficial
It was narrated from 'Amir bin Rabi'ah that the Prophet (ﷺ) said: "When any one of you sees a funeral and is not walking with it, let him stand up until it has passed him, or until (the body) is placed (in the grave) before if passes him."
Source: sunnah.com
Other translations
Arabic
أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا اللَّيْثُ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، عَنْ عَامِرِ بْنِ رَبِيعَةَ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ " إِذَا رَأَى أَحَدُكُمُ الْجَنَازَةَ فَلَمْ يَكُنْ مَاشِيًا مَعَهَا فَلْيَقُمْ حَتَّى تُخَلِّفَهُ أَوْ تُوضَعَ مِنْ قَبْلِ أَنْ تُخَلِّفَهُ " .
sunnah.com