Sunan Ibn Majah, Hadith 2129 ⓘ
It was narrated that 'Umar bin Khattab said: "I made a vow during the Ignorance period and I asked the Prophet (about it) after I became Muslim. He told me to fulfill my vow."
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا حَفْصُ بْنُ غِيَاثٍ، عَنْ عُبَيْدِ اللَّهِ بْنِ عُمَرَ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، عَنْ عُمَرَ بْنِ الْخَطَّابِ، قَالَ نَذَرْتُ نَذْرًا فِي الْجَاهِلِيَّةِ فَسَأَلْتُ النَّبِيَّ ـ صلى الله عليه وسلم ـ بَعْدَ مَا أَسْلَمْتُ فَأَمَرَنِي أَنْ أُوفِيَ بِنَذْرِي .
also referenced under: Book 11, Hadith 2210 (In-Book) & Volume 3, Book 11, Hadith 2129 (DarusSalam)