Musnad Abu Ya'la al-Mawsili 155:132

And from Abu Hurairah, he said: The Messenger of Allah, peace and blessings of Allah be upon him, was asked about the Night of Decree (Laylat al-Qadr). He said: "I was shown it, then I was made to forget it, and perhaps that is better for them. But seek it in the last ten nights of Ramadan."