Musnad Ahmad 31:590

Isma'il narrated to us, Hisham al-Dastawa'i informed us, from Yahya ibn Abi Kathir, from Abu Salamah, who said: We discussed the Night of Decree (Laylat al-Qadr) among a group of Quraysh. So I went to Abu Sa'id al-Khudri, and he was a friend of mine, and I said: "Come out with us to the palm trees." He went out, and he was wearing a *khamisah* (a square garment) of his. I said: "Did you hear the Messenger of Allah, peace and blessings of Allah be upon him, mention the Night of Decree?" He said: "Yes. We performed *i'tikaf* (seclusion in the mosque) with the Messenger of Allah, peace and blessings of Allah be upon him, during the middle ten days of Ramadan. The Messenger of Allah, peace and blessings of Allah be upon him, addressed us on the morning of the twentieth and said: 'I was shown the Night of Decree, then I was made to forget it'—or he said: 'I forgot it'—'so seek it in the last ten nights, in the odd-numbered ones. For I saw that I was prostrating in water and mud. So whoever performed *i'tikaf* with the Messenger of Allah, peace and blessings of Allah be upon him, let him return.'" We returned, and we did not see a single cloud in the sky. Then a cloud came, and it rained upon us until the roof of the mosque leaked, and it was made of palm fronds. The prayer was established, and I saw the Messenger of Allah, peace and blessings of Allah be upon him, prostrating in the water and mud, until I saw the trace of the mud on his forehead.