Musannaf Ibn Abi Shayba 6:440

Muhammad ibn Fudayl narrated to us, from Yazid ibn Abi Ziyad, from Abd al-Rahman ibn Abi Layla, from Ibn Abza, who said: I was at a funeral, and Abu Bakr and Umar were in front of it, and Ali was walking behind it. He said: So I came to Ali and said to him, "Is walking behind it better, or walking in front of it? For I see you walking behind it, while these two are walking in front of it." He said: So he said to me, "We have indeed known that walking behind it is better than in front of it, like the prayer of the congregation over the individual. However, they are both walking, and they like to make it easy for the people."