Sahih Bukhari
Fighting for the Cause of Allah (Jihaad)
Volume: 004 / Book Number: 052 / Hadith Number: 259
Narrated 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."