Mukhtasar Sahih Bukhari — Hadith 975

It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Noble Prophet (peace and blessings be upon him) said: "Seek Laylat al-Qadr in the last ten nights of Ramadan when nine, seven, or five nights remain (that is, on the twenty-first, twenty-third, and twenty-fifth nights)."