Mishkat al-Masabih #249
Knowledge
The Arabic includes the chain of narration (sanad) and the collector's commentary; the English translation shows the Prophet's words only.
وَعَنِ الْأَعْمَشِ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «آفَةُ الْعِلْمِ النِّسْيَانُ وَإِضَاعَتُهُ أَنْ تُحَدِّثَ بِهِ غَيْرَ أَهْلِهِ» . رَوَاهُ الدَّارِمِيُّ مُرْسلا
Al-A‘mash reported God’s messenger as saying, "The calamity
which affects knowledge is forgetfulness, and wasting it is to convey it to
those who are unworthy of it.” Darimi transmitted it in mursal form.
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About Mishkat al-Masabih
This hadith is #249 from Mishkat al-Masabih, in the Book of Knowledge.