Ibn `Umar (radiallahu anhuma) reported: The Messenger of Allah (salallahu alayhi wa sallam) used to observe I`tikaf in the last ten days of Ramadan.
[Al-Bukhari and Muslim].

'Aishah (radiallahu anha) reported: The Prophet (salallahu alayhi wa sallam) used to engage himself in I`tikaf (seclusion for prayers) in the mosque during the last ten nights of Ramadan till he passed away; thereafter, his wives followed this practice after him.
[Al-Bukhari and Muslim].

Abu Hurairah (radiallahu anh) reported: The Prophet (salallahu alayhi wa sallam) used to observe I`tikaf every year (during Ramadan) for ten days; in the year in which he passed away, he observed I`tikaf for twenty days.
[Al-Bukhari].

I`tikaf helps to enhance one's spiritual growth and to strive hard to worship Allah better as there is nothing to distract one's attention from this noble pursuit.