It is narrated from Sayyiduna Ibn Umar, may Allah be pleased with them both, that the Prophet, peace and blessings be upon him, said: "Whoever seeks Laylat al-Qadr should seek it in the last ten nights; if he is unable or becomes weak, then he should not be overcome in the last seven nights."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 5031