What was in the letter the Messenger (saw) sent to Harith the ruler of Damascus?

The Messenger (saw) sent the following letter to Harith bin Abi Shamir al-Ghassani, King of Damascus: "In the Name of Allah, the Most Beneficent, the Most Merciful. From Muhammad, Messenger of Allah to Harith bin Abi Shamir. Peace be upon him who follows true guidance, believes in it and regards it as true. I invite you to believe in Allah Alone with no associate, thence after your kingdom will remain yours." Shuja bin Wahab, the Messenger’s companion, carried his message to Harith who upon hearing the letter was madly infuriated. He threw the letter on the floor and uttered: “Who can take my kingdom away from me? I will march to him (i.e., fight him) even if he is in the Yemen.” He immediately started to mobilize his forces. Harith was no more than a governor whose jurisdiction extended only over his Arabian tribe, which was subjugated by the Byzantine rulers. Nevertheless, he was so boastful that he did not think anyone could take his kingdom from him. Therefore, his attitude was far more arrogant than his master Hercules. He wrote to Hercules, making known his intention to march with his forces against the Messenger (saw). Hercules wrote back from Jerusalem ordering him not to proceed with his intention calling him to join him in Jerusalem. Shuja bin Wahab reported to the Messenger (saw) that Harith had a Byzantine commander of his palace guard named Marri, a Christian. He questioned Shuja about the Messenger and his message. When Shuja explained that to him, his eyes were full of tears, he said: “I have read plain references to this Messenger in the Bible, but I thought that he would appear in Syria. However, I believe in him as Allah’s Messenger, but I fear that I shall be killed by Harith if I were to declare my belief.” He asked Shuja to convey his respects to the Messenger (saw) and to tell him that he was a believer in him and his message. (Ibn Qayyim, Za’ad al-Ma’ad)