Narrated 'Umar (RA):

[concerning his consultation with the Prophet] He said: I said: O Allah's Messenger (ﷺ), I made a vow in the Jahiliyya times to spend a night in devotion (I'tikaf) in the Sacred Mosque. He (the Prophet) said: "Fulfill your vow." [Agreed upon]. al-Bukhari added in another narration: "Then, spend a night in devotion (in the Sacred Mosque)."

وَعَنْ عُمَرَ ‏- رضى الله عنه ‏- قَالَ: { قُلْتُ: يَا رَسُولَ اَللَّهِ! إِنِّي نَذَرْتُ فِي اَلْجَاهِلِيَّةِ; أَنْ أَعْتَكِفَ لَيْلَةً فِي اَلْمَسْجِدِ اَلْحَرَامِ.‏ قَالَ:

فَأَوْفِ بِنَذْرِكَ

} مُتَّفَقٌ عَلَيْهِ [1]‏ وَزَادَ اَلْبُخَارِيُّ فِي رِوَايَةٍ [2]‏ { فَاعْتَكَفَ لَيْلَةً } [3]‏ .‏