Musnad Abu Ya'la al-Mawsili 244:17

Abu Ya'qub Ishaq ibn Abi Isra'il narrated to us, al-Nadr ibn Shumayl narrated to us, Ibn 'Awn narrated to us, from 'Umayr ibn Ishaq, who said: Ja'far sought permission from the Messenger of Allah (ﷺ), and said: "Permit me to go to a land where I may worship Allah, not fearing anyone." So he permitted him, and he went to the Negus. He said: 'Amr ibn al-'As narrated to me, saying: "When I saw his position, I envied him. I said: 'By Allah, I will surely cause him and his companions to be killed.' I went to the Negus and entered upon him with him, and I said: 'In your land is a man whose cousin is in our land, and he claims that there is no god for the people except one God. And by Allah, if you do not kill him and his companions, I will never bring this seed [of trade/goods] to you ever again, neither I nor any of my companions.'" He said: "Call him." I said: "He will not come with me." So he sent a messenger with me. He said: "He came, and when he reached the door, I called out: 'Give permission to 'Amr ibn al-'As.' But he called out from behind me: 'Give permission to 'Ubayd Allah [Ja'far].'" He said: "He heard his voice and gave him permission before me." He said: "He entered with his companions." He said: "Then he gave me permission, and I entered, and there he was, sitting." He said: "He mentioned where his seat was on the throne," and he mentioned the rest of the hadith in full.