Types of I`tikaaf (Hadith No. 1743)

Bismillah.
Volume 3, Book 33, Number 248:
Narrated Ibn 'Umar (radiallaahu `anhu):
Umar (radiallaahu `anhu) asked the Prophet (sallallaahu `alayhi wasallam) "I vowed in the Pre-lslamic period of ignorance to stay in Itikaf for one night in Al-Masjid al-Haram." The Prophet (sallallaahu `alayhi wasallam) said to him, "Fulfill your vow."
There are three main types of I`tikaaf:
  1. WAJIB: To vow or pledge to make I'tikaaf (on a fixed day) for the sake of Allah upon the fulfilment of some wish or desire.
  2. SUNNAT-MUAKKADAH: To reside the last ten nights and days of Ramadhan in the Masjid is Sunnat-Muakkadah, i.e. If a person from the community fulfills the obligation of I'tikaaf, the entire community will be absolved of this sacred duty. Otherwise all the residents will be sinful of neglecting this Sunnah.
  3. MUSTAHAB OR NAFL: This I'tikaaf can be for any amount of time, even for a few minutes.