Musnad Ahmad 32:1508

'Abd al-Wahhab narrated to us, he said: Sa'id was asked about the Night of Decree (Laylat al-Qadr), and he informed us from Qatadah, from Anas, that the Prophet of Allah, peace and blessings of Allah be upon him, said: "Seek it in the last ten nights, on the ninth, the seventh, and the fifth."