Narrated Thawban: An animal was brought to the Messenger of Allah (ﷺ) while he was going with a funeral. He refused to ride on it. When the funeral was away, the animal was brought to him and he rode on it. He was asked about it. He said: The angels were on their feet. I was not to ride while they were walking. When they went away, I rode.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا يَحْيَى بْنُ مُوسَى الْبَلْخِيُّ، أَخْبَرَنَا عَبْدُ الرَّزَّاقِ، أَخْبَرَنَا مَعْمَرٌ، عَنْ يَحْيَى بْنِ أَبِي كَثِيرٍ، عَنْ أَبِي سَلَمَةَ بْنِ عَبْدِ الرَّحْمَنِ بْنِ عَوْفٍ، عَنْ ثَوْبَانَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم أُتِيَ بِدَابَّةٍ وَهُوَ مَعَ الْجَنَازَةِ فَأَبَى أَنْ يَرْكَبَهَا فَلَمَّا انْصَرَفَ أُتِيَ بِدَابَّةٍ فَرَكِبَ فَقِيلَ لَهُ فَقَالَ " إِنَّ الْمَلاَئِكَةَ كَانَتْ تَمْشِي فَلَمْ أَكُنْ لأَرْكَبَ وَهُمْ يَمْشُونَ فَلَمَّا ذَهَبُوا رَكِبْتُ " .
sunnah.com