From Sahih Bukhari (Hadith 2804):
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 (ﷺ) ) 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
Reference: Bukhari 2804