Mishkat al-Masabih 8 The Excellent Qualities of the Qur’an كتاب فضائل القرآن 58

وَعَنِ ابْنِ عُمَرَ رَضِيَ اللَّهُ عَنْهُمَا قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «إِنَّ هَذِهِ الْقُلُوبَ تَصْدَأُ كَمَا يَصْدَأُ الْحَدِيدُ إِذَا أَصَابَهُ الْمَاءُ» . قِيلَ يَا رَسُولَ اللَّهِ وَمَا جِلَاؤُهَا؟ قَالَ: «كَثْرَةُ ذِكْرِ الْمَوْتِ وَتِلَاوَةِ الْقُرْآنِ» . رَوَى الْبَيْهَقِيُّ الْأَحَادِيثَ الْأَرْبَعَةَ فِي شُعَبِ الْإِيمَانِ

Ibn ‘Umar reported God’s messenger as saying, “These hearts become rusty just as iron does when water gets to it.” On being asked what could clear them he replied, “A great amount of remembrance of death and recitation of the Qur’ān.” Transmitted by Baihaqī in Shu’ab al-īmān .