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