It is essential that during worship, a person remains conscious that they are performing the act in pursuit of reward. When one is mindful of this, they are more likely to carry out the action in the best possible manner. Intellect naturally guides a person to understand that the reward corresponds to the quality of the deed—if it is performed well, the reward will be greater; if it is lacking, the reward will be less. I believe this is an issue many people overlook, as they fail to pay proper attention to this important aspect.

Tafsir Surah Az-Zumar pg.27