It is narrated from Hazrat Abu Saeed Khudri (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: Seek the Night of Qadr in the last ten nights of Ramadan, when nine nights remain, or seven, or five, or three.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 11679