Ibn al-Qayyim (may Alllaah have mercy upon him) said:
Beware of enabling Shaytaan to establish himself in the very home of your thoughts and intentions, as he will corrupt them in such a manner that will make difficult its correction thereafter. He will cast all sorts of whisperings and harmful thoughts at you and he will prevent you from thinking about what may benefit you. And it is you who have aided him against yourself by empowering him over your heart and thoughts, and he then placed you in possession of such thoughts.
- Source: Al-Fawaa’id | Transcribed from: Causes Behind the Increase and Decrease of Eemaan | Shaykh ‘Abdur-Razzaaq