What is the legal (Shari'ah) meaning of I'tikaf?

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In the Sharee'ah, I'tikaf is residing in the mosque according to a specified manner, and it is an act of worship and obedience. This is supported by the Divine command: "And [remember], We appointed the House (Kaaba) a place of return for the people and a sanctuary. And take, O believing people, from the standing place of Abraham a place of prayer" (Quran 2:125), and also: "And do not have relations with them while you are staying in retreat in the mosques" (Quran 2:187). The practice of the Prophet, peace be upon him, in observing the last ten nights of Ramadan further establishes its legitimacy.