Musnad Abu Ya'la al-Mawsili 42:351

Zuhayr narrated to us, Isma'il bin Ibrahim narrated to us, al-Jurayri narrated to me, from Abu Nadrah, from Abu Sa'id al-Khudri, who said: The Messenger of Allah (ﷺ) 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 passed, he ordered his tent to be dismantled, so it was dismantled. Then it was made clear to him that it is in the last ten days, so he ordered the tent to be set up, and it was restored. He performed I'tikaf during the last ten days. He then went out to the people and said: "O people, the Night of Decree was made clear to me, so I came out to inform you of it. I saw two men arguing, and the Shaytan was with them, so I forgot it. 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: "We are more entitled to that than you." As for 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. You leave the one you call the twenty-third, and the one that follows it is the seventh. You leave the one you call the twenty-fifth, and the one that follows it is the fifth."