Fighting for the Cause of Allah (Jihaad)

Bukhari :: Book 4 :: Volume 52 :: Hadith 42

Narrated Ibn 'Abbas:

Allah's Apostle said, "There is no Hijra (i.e. migration) (from Mecca to Medina) after the Conquest (of Mecca), but Jihad and good intention remain; and if you are called (by the Muslim ruler) for fighting, go forth immediately.