Hadith 507010
Chapter 19: I'tikaf in Ramadan - كتاب الاعتكاف
وَحَدَّثَنِي زِيَادٌ، عَنْ مَالِكٍ، عَنْ هِشَامِ بْنِ عُرْوَةَ، عَنْ أَبِيهِ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " تَحَرَّوْا لَيْلَةَ الْقَدْرِ فِي الْعَشْرِ الأَوَاخِرِ مِنْ رَمَضَانَ " .
Ziyad related to me from Malik from Hisham ibn Urwa from his father that the Messenger of Allah, may Allah bless him and grant him peace, said, "Search for Laylat al-Qadr in the last ten days of Ramadan."
Reference Muwatta Malik 507010
USC-MSA web (English) reference : Book 19, Hadith 11