From Sahih Bukhari (Hadith 2681):
Narrated `Abdullah bin `Abbas:Abu Sufyan told me that Heraclius said to him, "When I inquired you what he (i.e. Muhammad) ordered you, you replied that he ordered you to establish the prayer, to speak the truth, to be chaste, to keep promises and to pay back trusts." Then Heraclius added, "These are really the qualities of a prophet
Reference: Bukhari 2681