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 (peace be upon him) 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.
Sahih al-Bukhari, Vol. 4, Book 56, Hadith 21