(1) باب اعْتِكَافِ الْعَشْرِ الأَوَاخِرِ مِنْ رَمَضَانَ
Ibn 'Umar (Allah be pleased with both of them) reported that the Messenger of Allah (ﷺ) used to observe i'tikaf in the last ten days of Ramadan.
Reference: Sahih Muslim 1171 a
In-book reference: Book 14, Hadith 1
USC-MSA web (English) reference(deprecated numbering scheme): Book 6, Hadith 2636