Mishkat al-Masabih 2 Knowledge كتاب العلم 61

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.