Hawzah News Agency – Prophet Jesus peace be upon him says:

إنّ الدابَّةَ إذا لم تُركَبْ ولم تُمتَهَنْ وتُستَعمَلْ لَتَصعُبُ ويَتَغَيَّرُ خُلقُها، وكذلكَ القُلوبُ إذا لم تُرَقَّقْ بذِكرِ المَوتِ ويَتبَعُها دُؤوبُ العِبادَةِ تَقسُو وتَغلُظُ.

'When an animal is not mounted, trained or used it will become hard to use and its character will change. The same goes for the hearts - if they are not made soft with the remembrance of death, followed by diligent worship, they will harden and become harsh.'

Source: Bihar al-Anwar, v. 14, p. 309, no. 17