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