Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) said: Who knows when Laylat al-Qadr is? Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said: The Messenger of Allah (peace and blessings be upon him) said: "It is in the ten (last nights): when nine have passed, or when seven remain."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8534