What was in the letter the Messenger (saw) sent to Caesar the ruler of Rome?

The Muslim envoy, Dihyah bin Khalifah al-Kalbi, was ordered to hand the letter over to king of Busra, who would in turn, send it to Caesar. بسم الله الرحمن الرحيم من محمد عبد الله ورسوله إلى هرقل عظيم الروم سلام على من اتبع الهدى أما بعد فإني أدعوك بدعاية الإسلام اسلم تسلم يؤتك الله أجرك مرتين فإن توليت فعليك إثم الأريسيين و(ياأهل الكتاب تعالوا إلى كلمة سواء بيننا وبيتكم ألا تعبد إلا الله ولا نشرك به شيئاً ولا يتخذ بعضنا بعضاً آرياباً من دون الله فإن تولوا فقولوا اشهدوا بأنا مسلمون "In the Name of Allah, the Most Beneficent, the Most Merciful. From Muhammad, the slave of Allah and His Messenger to Hercules, king of the Byzantines.Blessed are those who follow true guidance. I invite you to submit Islam so that you may live in security. If you come within the fold of Islam, Allah will give you double reward, but in case you turn your back upon it, then the burden of the sins of all your people shall fall on your shoulders: “Say (O Muhammad): ‘O people of the Scripture (Jews and Christians), come to a word that is just between us and you, that we worship none but Allah, and that we associate no partners with Him, and that none of us shall take others as Lords besides Allah.’ Then, if they turn away, say: ‘Bear witness that we are Muslims.’” (Qur’an 3:64) (Sahih al-Bukhari) Hercules received the Messenger’s envoy well but wanted to establish for himself the Messenger’s true nature. He ordered his aides to find any person from Arabia who happened to be in the area, so that he could question him about the Messenger. It so happened that Abu Sufyan, the Quraysh leader, was in Gaza and was brought to Hercules with a number of his compatriots. Hercules, through his interpreter, asked them first, “Who is the closest relative among you to this man who claims to be a Messenger?” Abu Sufyan told him that he was. Hercules asked him to come forward. He also placed the other Arabs just behind Abu Sufyan and told them that he was about to put some questions to him. Abu Sufyan’s testimony went as follows: "Muhammad descends from a noble family. No one of his family happened to assume kingship. His followers are those deemed weak with numbers ever growing. He neither tells lies nor betrays others, we fight him and he fights us but with alternate victory. He bids people to worship Allah Alone with no associate, and abandon our forefathers’ beliefs. He orders us to observe prayer, honesty and abstinence and maintain strong family ties." "Hercules, on hearing this testimony, turned to his translator bidding him to communicate to us his following impression which reveals full conviction in the truthfulness of Muhammad’s Messengerhood: ‘I fully realize Messengers come from noble families; he does not affect any previous example of Messengerhood. Since none of his ancestors was a monarch, we cannot then allege that he is a man trying to reclaim his father’s monarchy. So long as he does not tell lies to people, he is for the more reason, immune to telling lies as regards Allah. Concerning his followers being those deemed weak with numbers ever growing, it is something that goes in agreement with questions of faith until this latter assumes its full dimensions geographically and demographically. I have understood that no instance of apostasy has as yet appeared among his followers, and this points to the bliss of faith that finds its abode in the human heart. Betrayal, as I see, is alien to him because true messengers hold betrayal in abhorrence. Bidding worship of Allah with no associates, observance of prayer, honesty and abstinence and prohibition of paganism are traits bound to subject to him all my possessions. I already knew a Messenger must arise but it never occurred to me he will be an Arab from among you. If I was sure I would be faithful to him, I might hope to meet him, and if I were with him, I would wash his feet.’ Hercules then requested the Messenger’s letter to be read out aloud. The observations of the emperor and finally the definite and clear-cut exposition of the Islamic message could not but create a tense atmosphere amongst the clergy present at the court. Abu Sufyan said: “We were ordered to go out and whilst coming out, I said to my companions, ‘The matter of Ibn Abi Kabshah (i.e.,Muhammad) has become so prominent that even the king of Banu al-Asfar (i.e., the Romans) is afraid of him.’ So, I continued to believe Allah’s Messenger would be victorious, till Allah made me embrace Islam." (Sahih al-Muslim) This is the most authentic report of Hercules’ reaction when he received the message. There are other reports, less authentic, suggesting Hercules tried to persuade his bishops and his advisers to embrace Islam, but they were all unanimous (with one exception) in opposing him. One report suggests the exception was the Archbishop, who was killed when he declared he believed in the new Messenger. Whatever the truth about these reports, the fact remains Hercules did not adopt Islam, perhaps due to his fear he would lose his throne as a result. (Ibn Qayyim, Za’ad al-Ma’ad)