It is narrated from Sayyiduna Ka'b bin Malik (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) ascended the pulpit and said: "I ascended it having come to know (the date of) 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 / 16256