The cause of covetousness, according to Sidi Ahmad al-Zarruq, is heedlessness (ghafla). A person permits himself to forget that blessings are from God alone. No good or harm can come to one except by God's leave. This level of heedlessness is not a casual lapse of memory. People can become so terribly preoccupied with seeking things from other people, they become heedless of God's power and ownership. When this happens, a person opens his or her heart to all kinds of spiritual diseases. The Prophet (peace be upon him) said, "Know that if an entire nation were to gather together to benefit you with anything, it would benefit you only with something that God had already prescribed for you. And if [an entire nation] were to gather together to harm you, it would harm you only with something that God had already prescribed for you." [Tirmidhi]
When the topic of God's power is discussed, questions often arise about those who hold rancor in their hearts to the degree that they wish harm to come to others. There is real concern about the affliction these people can cause. It is necessary to remember that when a person is straight with God—observant of His commands, avoiding what He has prohibited, and going beyond the mere obligations and remembering Him often through litanies, voluntary acts of worship, and generosity in charity—the evil prayers of others will not prevail.
"Purification of the Heart" - Hamza Yusuf