Musnad Ahmad 31:89

Ismail ibn Ibrahim narrated to us, from Sa’id al-Jurayri, from Abu Nadra, from Abu Sa’id, who said: The Messenger of Allah (peace and blessings of Allah be upon him) performed I’tikaf during the middle ten days of Ramadan, seeking the Night of Decree (Laylat al-Qadr) before it was made clear to him. When they had passed, he ordered his structure to be dismantled, and it was dismantled. Then it was made clear to him that it is in the last ten days. So he ordered the structure to be rebuilt, and it was rebuilt. Then he performed I’tikaf during the last ten days. Then he went out to the people and said: "O people, the Night of Decree was made clear to me, and I came out to inform you of it, but two men came disputing, and with them was the Shaytan, so I forgot it. So seek it in the ninth, the seventh, and the fifth." I said: "O Abu Sa’id, you are more knowledgeable about the count than us." He said: "I am more entitled to that than you. What are the ninth, the seventh, and the fifth?" He said: "You leave the one you call the twenty-first, and the one that follows it is the ninth; and you leave the one you call the twenty-third, and the one that follows it is the seventh; and you leave the one you call the twenty-fifth, and the one that follows it is the fifth."