Among the characteristics of the month of Ramadan: i'tikaf (spiritual retreat in the mosque).
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The Shaykh : And among the virtues of this month is that Allah has prescribed for His servants i'tikaf (spiritual retreat in the mosque) in it. So i'tikaf is prescribed in this month and is not prescribed in any other month; that is, it is not recommended except in Ramadan, because the Prophet (peace and blessings be upon him) did not perform i'tikaf except in Ramadan, except for one year when he left off i'tikaf in the last ten days and made it up in the first ten days of Shawwal. As for other times, the Prophet (peace and blessings be upon him) did not perform i'tikaf outside of Ramadan. This is because the purpose of i'tikaf is to obtain the Night of Decree. Consequently, the Prophet (peace and blessings be upon him) used to perform i'tikaf in the first ten days of Ramadan, then he performed i'tikaf in the middle ten days, then he informed [the people] that the Night of Decree is in the last ten days, so he began to perform i'tikaf in the last ten days.