Musnad Ahmad 821:1

Muhammad ibn Ja‘far narrated to us, Hisham and Yazid narrated to us, he said: Hisham informed us, from Hafsah, from Abu al-‘Aliyah, from an Ansari man. Yazid said: From a man from the Ansar, who said: I went out from my family intending [to go to] the Prophet, may Allah bless him and grant him peace, and suddenly I was with him standing, and a man was with him, facing him. I thought that they had a need. The Ansari said: By Allah, the Messenger of Allah, may Allah bless him and grant him peace, stood for so long that I began to feel pity for the Messenger of Allah, may Allah bless him and grant him peace, because of the length of the standing. When he turned away, I said: O Messenger of Allah, the man stood with you until I began to feel pity for you because of the length of the standing. He said: "And did you see him?" I said: Yes. He said: "Do you know who he is?" I said: No. He said: "That was Gabriel. He did not cease to advise me regarding the neighbor until I thought that he would make him an heir." Then he said: "As for you, if you had greeted him, he would have returned the greeting to you."