Jami` at-Tirmidhi Book 10, Hadith 46
"The Messenger of Allah would walk in front of the funeral, as did Abu Bakr, Umar and Uthman."
"The Prophet, Abu Bakr, and Umar would walk in front of the funeral." Az-Zuhri said: "Salim informed me that his father would walk in front of the funeral."
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.'"