Hadith 507020
Chapter 19: I'tikaf in Ramadan - كتاب الاعتكاف
وَحَدَّثَنِي زِيَادٌ، عَنْ مَالِكٍ، عَنْ عَبْدِ اللَّهِ بْنِ دِينَارٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " تَحَرَّوْا لَيْلَةَ الْقَدْرِ فِي السَّبْعِ الأَوَاخِرِ " .
Ziyad related to me from Malik from Abdullah ibn Dinar from Abdullah ibn Umar that the Messenger of Allah, may Allah bless him and grant him peace, said. "Search for Laylat al-Qadr in the last seven days."
Reference Muwatta Malik 507020
USC-MSA web (English) reference : Book 19, Hadith 12