Al-Nawawi reported: Sa’id ibn Jubayr, may Allah have mercy on him, said, “A man will continue to have knowledge as long as he is learning. If he forsakes knowledge and assumes that he is sufficient and already has enough, he will become ignorant.”
Source: al-Majmū’ Sharḥ al-Muhadhab 1/56
عن النووي وَقَالَ سَعِيدُ بْنُ جُبَيْرٍ رحمه الله لَا يَزَالُ الرَّجُلُ عَالِمًا مَا تَعَلَّمَ فَإِذَا تَرَكَ الْعِلْمَ وَظَنَّ أَنَّهُ قَدْ اسْتَغْنَى وَاكْتَفَى بِمَا عِنْدَهُ فَهُوَ أَجْهَلُ مَا يَكُونُ
1/56 المجموع شرح المهذب