It is narrated from Sayyiduna Ka'b bin Ujrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) ascended the pulpit and said: "I ascended while I had been given knowledge of the Night of Decree (Laylat al-Qadr), then I was made to forget it; so seek it in the odd nights of the last ten."
Hadith Referenceالمعجم الكبير للطبراني / باب الكاف / 16412