Sahih Bukhari Volume 4, Book 52, Hadith Number 259.

Narated By Abu Huraira : Allah’s Apostle sent us in a mission (i.e. am army-unit) and said, “If you find so-and-so and so-and-so, burn both of them with fire.” When we intended to depart, Allah’s Apostle said, “I have ordered you to burn so-and-so and so-and-so, and it is none but Allah Who punishes with fire, so, if you find them, kill them.”