Musnad Ahmad 30:656

Ali ibn Ishaq narrated to us, from Ibn al-Mubarak, from Yunus, from al-Zuhri, from Abu Umamah: The Prophet (ﷺ) said: "The heart of the son of Adam becomes rusty like iron becomes rusty." It was said: "O Messenger of Allah, what is its polishing?" He said: "The recitation of the Quran and the remembrance of death."