Hadith: The Messenger of Allah (may Allah's peace and blessings be upon him) used to practice I‘tikāf in the last ten nights of Ramadan until he passed away; then, his wives followed this practice after him

عَنْ عَائِشَةَ رَضِيَ اللَّهُ عَنْهَا زَوْجِ النَّبِيِّ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ:

أَنَّ النَّبِيَّ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ كَانَ يَعْتَكِفُ العَشْرَ الأَوَاخِرَ مِنْ رَمَضَانَ، حَتَّى تَوَفَّاهُ اللَّهُ، ثُمَّ اعْتَكَفَ أَزْوَاجُهُ مِنْ بَعْدِهِ.
[صحيح] - [متفق عليه]

‘Ā'ishah (may Allah be pleased with her) reported that the Messenger of Allah (may Allah's peace and blessings be upon him) used to practice I‘tikāf in the last ten nights of Ramadan until he passed away; then, his wives followed this practice after him. Another wording reads: "The Messenger of Allah (may Allah's peace and blessings be upon him) used to practice I‘tikāf every year in Ramadan; after offering the Morning Prayer, he would enter his place of I‘tikāf."
Sahih/Authentic. - [Al-Bukhari]

Explanation

‘Ā'ishah (may Allah be pleased with her) reported that the Prophet (may Allah's peace and blessings be upon him) used to practice I‘tikāf in the last ten nights of Ramadan, seeking Laylat al-Qadr after knowing that it was in the last ten nights. He kept doing that every year in Ramadan until he died. ‘Ā'ishah (may Allah be pleased with her) pointed out in this Hadīth that the ruling regarding this issue is not abrogated or restricted to the Prophet (may Allah's peace and blessings be upon him) for his wives (may Allah be pleased with them) practiced I‘tikāf after his death. In the other wording, ‘Ā'ishah (may Allah be pleased with her) clarified that after offering the Fajr prayer, the Prophet (may Allah's peace and blessings be upon him) used to enter his place of I‘tikāf, to devote himself to worshiping and invoking his Lord, a state that could only be achieved by suspending one's relations with the creation.