Sahih Bukhari
Fighting for the Cause of Allah (Jihaad)
Volume: 004 / Book Number: 052 / Hadith Number: 79
Narrated Ibn 'Abbas:
On the day of the Conquest (of Mecca) the Prophet said, "There is no emigration after the Conquest but Jihad and intentions. When you are called (by the Muslim ruler) for fighting, go forth immediately." (See Hadith No. 42)