Musnad Ahmad 31:713

'Affan narrated to us, Hammam narrated to us, Yahya ibn Abi Kathir narrated to us, Abu Salamah ibn 'Abd al-Rahman ibn 'Awf narrated to me, he said: I went to Abu Sa'id al-Khudri. He said: I said, "Will you not go out with us to the palm trees so we may talk?" He said: So he went out. He said: I said, "Narrate to me what you heard the Messenger of Allah, peace and blessings of Allah be upon him, say regarding the Night of Decree (Laylat al-Qadr)." He said: The Messenger of Allah, peace and blessings of Allah be upon him, performed I'tikaf (seclusion) during the first ten days of Ramadan, and we performed I'tikaf with him. Then Gabriel came to him and said, "Indeed, that which you are seeking is ahead of you." So he performed I'tikaf during the middle ten days of Ramadan, and we performed I'tikaf with him. Then Gabriel came to him and said, "Indeed, that which you are seeking is ahead of you." When it was the morning of the twentieth of Ramadan, the Messenger of Allah, peace and blessings of Allah be upon him, stood up to deliver a sermon and said: "Whoever performed I'tikaf with the Messenger of Allah, let him return, for I was shown the Night of Decree, and it is in the last ten days of Ramadan on an odd night, and I was made to forget it. And I saw as if I were prostrating in mud and water." He said: We did not see anything in the sky—Hammam said: I think he said: a small cloud (qaza'ah), naming the cloud by name—then a cloud came. The roof of the mosque was made of palm fronds, and it rained upon us. The Messenger of Allah, peace and blessings of Allah be upon him, led us in prayer, and I saw the trace of mud and water on the forehead of the Messenger of Allah, peace and blessings of Allah be upon him, and on the tip of his nose, in confirmation of his vision.