The learning of the Qur'an by heart and the reciting of it repeatedly Sahih al-Bukhari 5033
Arabic hadith text
حَدَّثَنَا مُحَمَّدُ بْنُ الْعَلاَءِ، حَدَّثَنَا أَبُو أُسَامَةَ، عَنْ بُرَيْدٍ، عَنْ أَبِي بُرْدَةَ، عَنْ أَبِي مُوسَى، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ "تَعَاهَدُوا الْقُرْآنَ فَوَالَّذِي نَفْسِي بِيَدِهِ لَهُوَ أَشَدُّ تَفَصِّيًا مِنَ الإِبِلِ فِي عُقُلِهَا "
Translation
Narrated Abu Musa
The Prophet (ﷺ) said, "Commit yourself to the Qur'an, for by Him in whose Hand is my soul, it is surely more prone to break away than a camel in its bind.