There is no joy for the one who does not bear sadness, there is no sweetness for the one who does not have patience, there is no delight for the one who does not suffer, and there is no relaxation for the one who does not endure fatigue.

Repel the thought, for if you don't, it becomes an idea. So repel the idea, for if you don't it will become a desire. So fight against that(desire), for if you don't, it will become a determination and a passion. And if you don't repel that, it will become an action. And if you don't replace it with its opposite, it will become a constant habit. So at that point, it will be difficult for you to change it.

The hijrah to Allah includes abandoning what He hates and doing what He loves and accepts.

The heart becomes sick, as the body becomes sick, and its remedy is al-Tawbah (repentance) and protection [from transgression]. It becomes rusty as a mirror becomes rusty, and its clarity is obtained by remembrance. It becomes naked as the body becomes naked, and its beautification is al-Taqwa. It becomes hungry and thirsty as the body becomes hungry, and its food and drink are knowledge, love, dependence, repentance and servitude.

The sinner does not feel any remorse over his sins, that is because his heart is already dead

As you can taste a pot full of food with a spoon likewise someone's tounge can tell you about his heart.

And verily for everything that a slave loses there is a substitute, but the one who loses Allah will never find anything to replace Him.