Hadith 13:23
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 وَزَادَ اَلْبُخَارِيُّ فِي رِوَايَةٍ { فَاعْتَكَفَ لَيْلَةً }
Original Reference: Bulugh al-Maram 1396