Jami` at-Tirmidhi Book 10, Hadith 48
"We went with the Prophet (following) a funeral. He saw people riding so he said: 'Are you not ashamed? Indeed Allah's angels are on their feet, while you are on the backs of your beasts'"
From Abdullah bin Mas'ud who said: "We asked the Messenger of Allah about walking behind the funeral. He said: 'Less than a trot. For if he was good, then you will be hastening him to it (goodness), and if he was evil, then it id only an inhabitant of the Fire that is being taken away." The funeral is (to be) followed. The one who precedes it shall not have the reward of those who follow it.'"
"We were with the Prophet (following) the funeral of Ibn Ad-Dahbah, and he was on a horse that was walking quickly, and we were around him, and he was trotting."