Abu Dawud Book 2, Hadith Number 1378.

Narated By Abu Sa’id al-Khudri : The Prophet (PBUH) said: Seek it (laylat al-Qadr) in the last ten days of Ramadan. Seek it on the ninth, seventh and fifth night. I (Abu Nadrah) said: You know counting better than us, AbuSa’id. He said: Yes. I asked: What do you mean by the ninth, seventh and fifth night? He said: When the twenty-first night passes, the night which follows it is the night; when the twenty-third night passes, the night which follows it is the seventh; when the twenty-fifth passes, the night which follows it is the fifth.