Musnad Ahmad 26:560

Waki’ narrated to us, Hasan narrated to us, from Yahya ibn al-Harith, from Abu Majid al-Hanafi, from Ibn Mas’ud, who said: We asked the Messenger of Allah, peace and blessings of Allah be upon him, about walking with the funeral procession. He said: "It is less than a trot. The funeral procession is followed, and it does not follow."