Musannaf Ibn Abi Shayba 36:21

Ubayd Allah narrated to us, he said: Isra'il informed us, from Abu Ishaq, from Abu Maysarah, that when the Messenger of Allah (peace and blessings of Allah be upon him) would go out to the open country, he would hear someone calling him: "O Muhammad." When he heard the voice, he would depart fleeing. He would come to Khadijah and mention that to her, and he would say: "O Khadijah, I fear that something has affected my mind. When I go out to the open country, I hear someone calling me, but I do not see anything, so I depart fleeing, and then he is with me calling me." She said: "Allah would not do that to you. For as long as I have known you, you speak the truth, fulfill the trust, and maintain ties of kinship, so He would not do that to you." He kept that secret to Abu Bakr, and he was a companion of his during the Pre-Islamic period. Abu Bakr took his hand and went with him to Waraqah. He said: "And what is that?" He told him what Khadijah had told him. He came to Waraqah and mentioned that to him. Waraqah said: "Do you see [anything]?" He said: "No, but when I go out to the open country, I hear the call, but I do not see anything, so I depart fleeing, and then he is with me." He said: "Do not do that. When you hear it, stand firm until you hear what he says to you." When he went out to the open country, he heard the call: "O Muhammad." He said: "Labbayk (at your service)." He said: "Bear witness that there is no god but Allah, and bear witness that Muhammad is His servant and His Messenger." Then he said to him: "Say: 'All praise is due to Allah, Lord of the worlds, the Entirely Merciful, the Especially Merciful, Sovereign of the Day of Recompense,'" until he finished the Opening of the Book (Al-Fatihah). Then he came to Waraqah and mentioned that to him. Waraqah said to him: "Rejoice, then rejoice, then rejoice. For I bear witness that you are the Messenger whom Jesus gave glad tidings of, [saying]: 'A messenger who will come after me, his name is Ahmad.' So I bear witness that you are Ahmad, and I bear witness that you are Muhammad, and I bear witness that you are the Messenger of Allah. And you will soon be commanded to fight, and if you are commanded to fight while I am alive, I will surely fight with you." Waraqah died, and the Messenger of Allah (peace and blessings of Allah be upon him) said: "I saw the priest in Paradise, wearing green garments."