Hawzah News Agency – Imam al-Sadiq peace be upon him says:

إنَّ للقُلوبِ صَداءً كصَداءِ النُّحاسِ، فاجلُوها بالاستِغفارِ.

'Hearts have rust like the rust of copper so polish them with seeking forgiveness.'

Source: 'Uddat al-Dai, p. 249