كتاب الصيام
5  
Fasting
Bulugh al-Maram 700
’A’isha (RAA) narrated, "Whenever the Messenger of Allah (ﷺ) intended to make I'tikaf he would pray the morning prayer and then he would enter the place of his I’tikaf (i.e. the place, which he had prepared for his I'tikaf inside the mosque)." Agreed upon.
وَعَنْهَا قَالَتْ: { كَانَ رَسُولُ اَللَّهِ ‏- صلى الله عليه وسلم ‏-إِذَا أَرَادَ أَنْ يَعْتَكِفَ صَلَّى اَلْفَجْرَ, ثُمَّ دَخَلَ مُعْتَكَفَهُ } مُتَّفَقٌ عَلَيْهِ 1‏ .‏
Reference : Bulugh al-Maram 700
In-book reference : Book 5, Hadith 700
English translation : Book 5, Hadith 720
Hadith Text
Reference