Musnad Ahmad

Hazrat Abu Saeed Khudri (may Allah be pleased with him) narrates that once the Prophet (peace and blessings be upon him) performed i'tikaf during the middle ten days of Ramadan. The Prophet (peace and blessings be upon him) was searching for Laylat al-Qadr, and until that time it had not been made clear to him. When that ten days ended, the Prophet (peace and blessings be upon him) ordered his tent to be removed. Then it was informed to the Prophet (peace and blessings be upon him) that it would be in the last ten days, so by his order, his tent was set up again, and the Prophet (peace and blessings be upon him) also performed i'tikaf in the last ten days. Then he came out to the people and said: O people! I was informed about Laylat al-Qadr, and I came out to tell you, but two men came arguing, and Satan was with them, so its exact determination was made to be forgotten by me. Now you should seek it in the ninth, seventh, and fifth (nights). I said: O Abu Saeed! You know counting better than us. He said: I am more entitled to it than you. I asked about the meaning of the seventh, ninth, and fifth, so he said: the twenty-first and the night next to it is the ninth, the twenty-third and the night next to it is the seventh, and the twenty-fifth and the night next to it is the fifth.