Muwatta Malik 698
I'tikaf in Ramadan
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
Grading: Salim al-Hilali: Sahih
Text from the public-domain hadith-api dataset. Numbering follows that dataset and can differ by a few from some printed editions — check the wording, not only the number.