Sahih Bukhari Volume 3, Book 48, Hadith Number 846.

Narated By Abdullah bin Abbas : Abu Sufyan told me that Heraclius said to him, “When I enquired 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.”