From Sahih Bukhari (Hadith 7196):
Narrated `Abdullah bin `Abbas:That Abu Sufyan bin Harb told him that Heraclius had called him along with the members of a Quraish caravan and then said to his interpreter, "Tell them that I want to ask this (Abu Sufyan) a question, and if he tries to tell me a lie, they should contradict him." Then Abu Sufyan mentioned the whole narration and said that Heraclius said to the inter Peter, "Say to him (Abu Sufyan), 'If what you say is true, then he (the Prophet) will take over the place underneath my two feet
Reference: Bukhari 7196