Musnad Ahmad 1153:1975

Yazid narrated to us, he said: Dawud informed us, from Amir, from Masruq, who said: I was reclining near Aisha, and she said: "O Abu Aisha, am I the first one to ask the Messenger of Allah, peace and blessings of Allah be upon him, about this?" He said: "That is Gabriel; I have not seen him in the form in which he was created, except twice. I saw him descending from the sky, his immense size blocking what is between the heaven and the earth."