If somebody vowed during the Pre-Islamic Period of Ignorance, and then he embraces Islam
حَدَّثَنَا مُحَمَّدُ بْنُ مُقَاتِلٍ أَبُو الْحَسَنِ، أَخْبَرَنَا عَبْدُ اللَّهِ، أَخْبَرَنَا عُبَيْدُ اللَّهِ بْنُ عُمَرَ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، أَنَّ عُمَرَ، قَالَ يَا رَسُولَ اللَّهِ إِنِّي نَذَرْتُ فِي الْجَاهِلِيَّةِ أَنْ أَعْتَكِفَ لَيْلَةً فِي الْمَسْجِدِ الْحَرَامِ. قَالَ " أَوْفِ بِنَذْرِكَ ".
Narrated Ibn `Umar: `Umar said "O Allah's Messenger! I vowed to perform I`tikaf 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 `Umar said to the lady, "Pray on her behalf." Ibn `Abbas said the same.