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 regarding the last seven nights."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 5443
Hadith Gradingحکم دارالسلام:إسناده صحيح ، خ : 2015، م :1165.