Musannaf Ibn Abi Shayba 36:94

Hatim ibn Isma'il narrated to us, from Ya'qub, from Ja'far ibn 'Amr, who said: The Messenger of Allah (peace and blessings of Allah be upon him) sent four men to four directions: a man to Chosroes, a man to Caesar, a man to the Muqawqis, and he sent 'Amr ibn Umayyah to the Negus. Each man among them woke up speaking the language of the people to whom he was sent. When 'Amr ibn Umayyah came to the Negus, he found a small door for them through which they would enter while bowing. When 'Amr saw that, he turned his back and retreated. This distressed the Abyssinians in their gathering with the Negus, to the point that they intended to harm him. They said to the Negus, "This man did not enter as we entered." The Negus said, "What prevented you from entering as they entered?" He replied, "We do not do this for our Prophet, and if we were to do it for anyone, we would have done it for him." The Negus said, "He has spoken the truth. Leave him." They said to the Negus, "This man claims that Jesus is a slave." The Negus said, "What do you say about Jesus?" He replied, "The Word of Allah and His Spirit." The Negus said, "Jesus could not have exceeded that."