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.
Qurʾān Bukhārī Muslim Nasāʾī Abū Dāwūd Tirmidhī Ibn Mājah Mālik Aḥmad Dārimī Ḥākim Ibn Ḥibbān Ṭabarānī Nasāʾī's Kubrá Bayhaqī al-Luʾluʾ wa-al-Marjān Ibn Rajab's Fifty Riyāḍ al-Ṣāliḥīn Aḥmad's Zuhd Suyūṭī
ahmad-zuhd:853 – ʿAbdullāh from my father > Wakīʿ > al-Masʿūdī > al-Qāsim b. ʿAbd al-Raḥman Wa-al-Ḥasan b. Saʿd

[Machine] "Abdullah, I believe that a person who forgets knowledge that he once possessed, commits a mistake."  

الزهد لأحمد:٨٥٣ – حَدَّثَنَا عَبْدُ اللَّهِ حَدَّثَنَا أَبِي حَدَّثَنَا وَكِيعٌ حَدَّثَنَا الْمَسْعُودِيُّ عَنِ الْقَاسِمِ بْنِ عَبْدِ الرَّحْمَنِ وَالْحَسَنِ بْنِ سَعْدٍ قَالَ

عَبْدُ اللَّهِ إِنِّي لَأَحْسَبُ الرَّجُلَ يَنْسَى الْعِلْمَ كَانَ يَعْلَمُهُ بِالْخَطِيئَةِ يَعْمَلُهَا  

See similar narrations below:

Collected by Dārimī, Ṭabarānī
darimi:388 – Yaʿlá > al-Masʿūdī > al-Qāsim

[Machine] To me, Abdullah says,"Verily, I reckon that a man forgetting knowledge that he knew is a sin that he commits."  

الدارمي:٣٨٨ – أَخْبَرَنَا يَعْلَى حَدَّثَنَا الْمَسْعُودِيُّ عَنِ الْقَاسِمِ قَالَ قَالَ

لِي عَبْدُ اللَّهِ «إِنِّي لَأَحْسَبُ الرَّجُلَ يَنْسَى الْعِلْمَ كَانَ يَعْلَمُهُ لِلْخَطِيئَةِ كَانَ يَعْمَلُهَا»  

tabarani:8930 – ʿAlī b. ʿAbd al-ʿAzīz > Abū Nuʿaym > al-Masʿūdī > al-Qāsim

[Machine] Abdullah: "Verily, I believe that a person forgets knowledge whenever they commit a sin that they have learned."  

الطبراني:٨٩٣٠ – حَدَّثَنَا عَلِيُّ بْنُ عَبْدِ الْعَزِيزِ ثنا أَبُو نُعَيْمٍ ثنا الْمَسْعُودِيُّ عَنِ الْقَاسِمِ قَالَ

عَبْدُ اللهِ «إِنِّي لَأَحْسِبُ الرَّجُلَ يَنْسَى الْعِلْمَ كُلَّمَا يَعْلَمُهُ الْخَطِيئَةَ يَعْمَلُهَا»