Fiqh-us-Sunnah Volume 3, Zakaat and Fasting, Fiqh 3.147B.

I’tikaf is of two types: sunnah and obligatory. The sunnah i’tikaf is that which the Muslim performs to get closer to Allah by following the actions of the Prophet, upon whom be peace, especially during the last ten days of Ramadan. The obligatory i’tikaf is that which the person makes obligatory upon himself. This may be done, for example, by an oath: “For Allah I must make i’tikaf,” or by a conditional oath: “If Allah cures me, I shall make i’tikaf…” In Sahih al-Bukhari it is reported that the Prophet, upon whom be peace, said: “Whoever makes an oath to obey Allah should be obedient to Him.” ‘Umar said: “O Messenger of Allah, I made an oath to perform i’tikaf one night in the mosque at Makkah.” The Prophet, upon whom be peace, said: “Fulfil your oath.”