(1) باب اعْتِكَافِ الْعَشْرِ الأَوَاخِرِ مِنْ رَمَضَانَ
'A'isha (Allah be pleased with her) reported that the Messenger of Allah (may peace he upon him) used to observe i'tikif in the last ten days of Ramadan till Allah called him back (to his heavenly home). Then his wives observed i'tikaf after him.
Reference: Sahih Muslim 1172 c
In-book reference: Book 14, Hadith 5
USC-MSA web (English) reference(deprecated numbering scheme): Book 6, Hadith 2640