Caution: Translations of Quran and Ḥadīth may lead to possible misapplications and misinterpretations. This site is intended for students of sacred knowledge that are proficient in comprehending classical Arabic and have a strong foundation in Islamic sciences. Also note that religious injunctions rely on several aspects beyond what one may glean through reading individual aḥādīth.
suyuti:2-1380bʿMr b. al-Khṭāb
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السيوطي:٢-١٣٨٠b

"عن عمر بن الخطاب أَنَّه كُتِب إليه في رجلٍ قِيلَ لَهُ: متَى عَهْدُكَ بالنِّساء؟ فقال: البارحةَ، قيل: بِمَنْ؟ قَالَ: أم مَثْوَى، فقيل له: قد هَلكت، قال: ما علمتُ أن اللَّه حرَّم الزنا، فكتبَ عمر أن يُسْتَحْلَف ما علم أَن اللَّه حرم الزِّنا ثم يُخلَّى سبيلُه".  

أبو عبيد في الغريب، [ق] البيهقى في السنن

See similar narrations below:

Collected by Bayhaqī
bayhaqi:17066Abū ʿAbdullāh al-Sulamī > Abū al-Ḥasan al-Kārizī > ʿAlī b. ʿAbd al-ʿAzīz > Abū ʿUbayd > Marwān b. Muʿāwiyah And Yazīd > Ḥumayd > Bakr b. ʿAbdullāh > ʿUmar b. al-Khaṭṭāb > Kutib

[Machine] It was written to him about a man who was asked, "When did you start dealing with women?" He said, "Yesterday." They asked, "With whom?" He said, "My mother." He was told, "You are already dead." He said, "I did not know that Allah had forbidden adultery." So Umar ibn al-Khattab wrote that he should be sworn to not pursue what he did not know Allah had forbidden, and then he should be freed.  

البيهقي:١٧٠٦٦أَخْبَرَنَا أَبُو عَبْدِ اللهِ السُّلَمِيُّ أنبأ أَبُو الْحَسَنِ الْكَارِزِيُّ أنبأ عَلِيُّ بْنُ عَبْدِ الْعَزِيزِ قَالَ قَالَ أَبُو عُبَيْدٍ ثنا مَرْوَانُ بْنُ مُعَاوِيَةَ وَيَزِيدُ عَنْ حُمَيْدٍ عَنْ بَكْرِ بْنِ عَبْدِ اللهِ عَنْ عُمَرَ بْنِ الْخَطَّابِ ؓ

أَنَّهُ كُتِبَ إِلَيْهِ فِي رَجُلٍ قِيلَ لَهُ مَتَى عَهْدُكَ بِالنِّسَاءِ؟ فَقَالَ الْبَارِحَةَ قِيلَ بِمَنْ؟ قَالَ أُمِّ مَثْوَايَ فَقِيلَ لَهُ قَدْ هَلَكْتَ قَالَ مَا عَلِمْتُ أَنَّ اللهَ حَرَّمَ الزِّنَا فَكَتَبَ عُمَرُ ؓ أَنْ يُسْتَحْلَفَ مَا عَلِمَ أَنَّ اللهَ حَرَّمَ الزِّنَا ثُمَّ يُخَلَّى سَبِيلُهُ