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
darimi:333 – Bishr b. al-Ḥakam > ʿAbdullāh b. Rajāʾ > Hishām > al-Ḥasan

[Machine] They used to say, "The death of a scholar is a gap in Islam that cannot be filled. No matter the difference between night and day,".  

الدارمي:٣٣٣ – أَخْبَرَنَا بِشْرُ بْنُ الْحَكَمِ حَدَّثَنَا عَبْدُ اللَّهِ بْنُ رَجَاءٍ عَنْ هِشَامٍ عَنِ الْحَسَنِ قَالَ

كَانُوا يَقُولُونَ «مَوْتُ الْعَالِمِ ثُلْمَةٌ فِي الْإِسْلَامِ لَا يَسُدُّهَا شَيْءٌ مَا اخْتَلَفَ اللَّيْلُ وَالنَّهَارُ»  

See similar narrations below:

Collected by Aḥmad's Zuhd, Suyūṭī
ahmad-zuhd:1473 – ʿAbdullāh from my father > Abū ʿUbaydah > Hishām > al-Ḥasan

[Machine] "They used to say that the death of a scholar is a gap in Islam that cannot be filled. The night and day may differ, but it remains unchanged."  

الزهد لأحمد:١٤٧٣ – حَدَّثَنَا عَبْدُ اللَّهِ حَدَّثَنِي أَبِي حَدَّثَنَا أَبُو عُبَيْدَةَ حَدَّثَنَا هِشَامٌ عَنِ الْحَسَنِ قَالَ

كَانُوا يَقُولُونَ مَوْتُ الْعَالِمِ ثَلْمَةٌ فِي الْإِسْلَامِ لَا يَسُدُّهَا شَيْءٌ مَا اخْتَلَفَ اللَّيْلُ وَالنَّهَارُ