Musnad Ahmad 30:2716

Hajjaj narrated to us, from Layth, who said: Bukayr ibn Abdullah narrated to me, from Sulayman ibn Yasar, from Abu Hurayrah, that he said: The Messenger of Allah (peace and blessings of Allah be upon him) sent us on an expedition and said: "If you find so-and-so and so-and-so"—referring to two men from Quraysh—"then burn them with fire." Then, when we intended to depart, the Messenger of Allah (peace and blessings of Allah be upon him) said: "I had commanded you to burn so-and-so and so-and-so with fire, but none punishes with fire except Allah. So if you find them, kill them."