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ALLAHU AKBAR!
Ibn al-Jawzi (may Allaah have mercy on him) said in Talbees Iblees (p. 52):
Iblees tricks people in whatever way he can, and his power over them increases and decreases according to their alertness or negligence, their ignorance or knowledge.
Note that the heart is like a fortress, and the devils are always going around the fortress watching for the guards to become heedless. The guard should know all the gates of the fortress which he has been appointed to guard, and he should not let his attention slip for a moment, for the enemy never lets his attention slip.
A man said to al-Hasan al-Basri: Does Iblees sleep?
He said: If he slept, we would have a break! End quote.
Source: ISLAMQA