Sunan an-Nasa'i, Hadith 942 ⓘ
It was narrated from Ibn Umar that: The Messenger of Allah said: "The one who learns the Quran is like the owner of a hobbled camel. If he pays attention to it, he will keep it, but if he releases it, it will go away."
أَخْبَرَنَا قُتَيْبَةُ، عَنْ مَالِكٍ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " مَثَلُ صَاحِبِ الْقُرْآنِ كَمَثَلِ صَاحِبِ الإِبِلِ الْمُعَقَّلَةِ إِذَا عَاهَدَ عَلَيْهَا أَمْسَكَهَا وَإِنْ أَطْلَقَهَا ذَهَبَتْ " .
also referenced under: Book 11, Hadith 67 (In-Book) & Volume 2, Book 11, Hadith 943 (DarusSalam)