Hawzah News Agency – The merciful prophet of Islam, peace be upon him and his household says:

إنّ هذهِ القُلوبَ تَصدَأُ كما يَصدأُ الحَديدُ إذا أصابَهُ الماءُ. قيلَ: وما جَلاؤها؟ قالَ: كَثرَةُ ذِكرِ المَوتِ، وتِلاوَةِ القرآنِ.

'Verily these hearts rust like metal rusts when exposed to water.' He was asked, 'And what can polish them?' He said, 'Remembering death much, and reciting the Quran.'

Source: Kanz al-Ummal, no. 42130