Hadithcore
Mishkat al-Masabih, 2168

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 <i>Shu'ab al-īmān</i>.

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

References1 variant
In-Book Reference
Book 8, Hadith 58