What is the origin and Sunnah regarding staying overnight during I'tikaf: should it be by sleeping inside the mosque or outside of it?

I'tikaf is to remain in the mosque for the obedience of Allah. It is a Sunnah throughout the year, unless one makes it obligatory upon himself through a vow. Its minimum is what is customarily referred to as i'tikaf. If a person intends to observe i'tikaf for one day or more, he must remain in the mosque day and night and not leave except for a necessary need, such as relieving himself, eating, or drinking. The place where the one observing i'tikaf sleeps is the mosque, because his departure for other than a necessity invalidates the i'tikaf.