Bulugh al-Maram 1396
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 .‏‏1 ‏- صحيح.‏ رواه البخاري ( 2032 )‏، ومسلم ( 1656 )‏.‏‏2 ‏- ووقع في "أ" : "روايته".‏‏3 ‏- البخاري ( 2042 )‏.‏