ʿUmar said "O Messenger of Allah ﷺ! I vowed to perform Iʿtikaf for one night in Al-Masjid-al-Haram, during the Pre-Islamic Period of ignorance (before embracing Islam). "The Prophet ﷺ said, "Fulfill your vow." Ibn ʿUmar said to the lady, "Pray on her behalf." Ibn ʿAbbas said the same. (Using translation from Bukhārī 6697)   

قُلْتُ يَا رَسُولَ اللهِ إِنِّي نَذَرْتُ أَنْ أَعْتَكِفَ لَيْلَةً فِي الْمَسْجِدِ الْحَرَامِ وَقَدْ جَاءَ اللهُ بِالْإِسْلَامِ فَقَالَ «أَوْفِ بِنَذْرِكَ»  

O Messenger of Allah ﷺ, I vowed during the jahiliyyah to observe i’tikaf for one night in al-Masjid al-Haram. He said to him: “Fulfil your vow.” (Using translation from Aḥmad 255)   

أَنَّهُ سَأَلَ رَسُولَ اللهِ ﷺ قَالَ يَا رَسُولَ اللهِ إِنِّي نَذَرْتُ أَنْ أَعْتَكِفَ لَيْلَةً فِي الْمَسْجِدِ الْحَرَامِ فِي الْجَاهِلِيَّةِ قَالَ «أَوْفِ بِنَذْرِكَ»  

It was narrated from Ibn 'Umar that 'Umar had vowed -during the Jahiliyyah- to spend a day in 'Itikaf. He asked the Messenger of Allah about that, and he commanded him to perform the 'Itikaf. (Using translation from Nasāʾī 3822)  

أَنَّ عُمَرَ كَانَ قَدْ جَعَلَ عَلَيْهِ يَوْمًا يَعْتَكِفُهُ فِي الْجَاهِلِيَّةِ فَسَأَلَ رَسُولَ اللهِ ﷺ عَنْ ذَلِكَ «فَأَمَرَهُ أَنْ يَعْتَكِفَ»