Kitab al-Fawa’id by Imam Ibn al-Qayyim
One: increase and extravagance; to have more than one needs. This (extra) addition is the way which Satan uses to enter the heart. To protect oneself from that is to be careful not to fulfil all the needs of the soul like food, sleep, pleasure, or rest. Whenever you close this door, you will be safe from any enemy.
Two: negligence; whoever remembers Allah is protected by the fortress of remembering Him. When one neglects that, one will be opening the door of this fortress and the enemy will enter. It would then be difficult to drive him out.
Three: caring for what is not his business in all matters of life