'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. (IT)
English reference (IT): Book 5, Hadith 720 (IT)
Arabic reference (IT): Book 5, Hadith 700 (IT)