Fighting for the Cause of Allah (Jihaad)

Bukhari :: Book 4 :: Volume 52 :: Hadith 60

Narrated Abdullah bin Abbas:

That Abu Sufyan told him that Heraclius said to him, "I asked you about the outcome of your battles with him (i.e. the Prophet ) and you told me that you fought each other with alternate success. So the Apostles are tested in this way but the ultimate victory is always theirs.