Hawzah News Agency – It is narrated in al-Kafi that,

فيما ناجَى اللَّهُ عَزَّوجلَّ بهِ موسى‏ عليه السلام: يا موسى‏، لا تُطَوِّلْ في الدُّنيا أملَكَ فَيَقسُوَ قَلبُكَ، والقاسي القَلبِ مِنّي بَعيدٌ.

In one of Allah's intimate conversations with Prophet Moses (AS), He said, 'O Moses, do not prolong your hope in this world for your heart will turn hard, and hard hearts are far away from Me.'

Source: al-Kafi, v. 2, p. 329, no. 1