It is narrated from Hazrat Abu Bakrah (may Allah be pleased with him) that I heard the Prophet (peace and blessings be upon him) saying: Seek the Night of Qadr in the last ten nights of Ramadan—on the twenty-first night, the twenty-third night, the twenty-fifth night, the twenty-seventh night, or the last night.
Hadith Referenceمسند احمد / أول مسند البصريين / 20376