Musannaf Ibn Abi Shayba 36:93

'Abd al-Rahim ibn Sulayman narrated to us, from 'Abd al-Rahman ibn Harmala al-Aslami, who said: I heard Sa'id ibn al-Musayyib saying: The Messenger of Allah (may Allah bless him and grant him peace) wrote to Chosroes, Caesar, and the Negus: "To proceed: Come to a word common between us and you, that we worship none but Allah, and that we associate nothing 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." Sa'id said: Chosroes tore up the letter and did not look into it. The Prophet of Allah said: "He tore it up, and his nation shall be torn up." As for the Negus, he believed, and those who were with him believed, and he sent a gift of a garment to the Messenger of Allah (may Allah bless him and grant him peace). The Messenger of Allah (may Allah bless him and grant him peace) said: "Leave them alone, as long as they leave you alone." As for Caesar, he read the letter of the Messenger of Allah (may Allah bless him and grant him peace) and said: "This is a letter the like of which I have not heard since Solomon the Prophet. In the name of Allah, the Most Gracious, the Most Merciful." Then he sent for Abu Sufyan and al-Mughira ibn Shu'ba, who were merchants in his land. He asked them about some of the affairs of the Messenger of Allah (may Allah bless him and grant him peace), and he asked them who followed him. They said: "The women and the weak people followed him." He said: "Have you seen those who enter with him and then leave?" They said: "No." He said: "He is a Prophet; he shall surely possess what is beneath my feet. If I were with him, I would kiss his feet."