Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) narrates that the Messenger of Allah (peace and blessings be upon him) said: "Seek it; Laylat al-Qadr is in the last ten (nights) of Ramadan, when nine remain, or seven remain, or five remain."
And Khalid narrated from Ikrimah that Ibn Abbas (may Allah be pleased with them both) said: Seek it on the twenty-fourth.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8533