(This means) Steadfastness of heart and not being disturbed about the imaginary things that bad thoughts may bring to mind. For when a person gives in to his imagination and lets his mind be disturbed by these thoughts, such as fear of disease and the like, or anger and confusion stirred up by some grievous matter, or the expectation of bad things and the loss of good things, that will fill him with worries, distress, mental and physical illness and nervous breakdowns, which will have a bad effect on him and which causes a great deal of harm, as many 
people have seen. But when a person depends on Allah and puts his trust in Him, and does not give in to his imagination or let bad thoughts overwhelm him, and he relies on Allah and has hope of His bounty, that wards off his worries and distress, and relieves him of a great deal of mental and physical sickness. It gives indescribable strength, comfort and happiness to the heart. How many hospitals are filled with the mentally sick victims of illusions and harmful imagination; how often have these things had an effect upon the hearts of many strong people, let alone the weak ones; how often have they led to foolishness and insanity.
It should be noted that your life will follow your train of thought. If your thoughts are of things that will bring you benefit in your spiritual or worldly affairs, then your life will be good and happy. Otherwise it will be the opposite.
The person who is safe from all of that is the one who is protected by Allah and helped by Him to strive to achieve that which will benefit and strengthen the heart and ward off anxiety. Allah says:
“And whosoever puts his trust in Allaah, then He will suffice him” [al-Talaaq 65:3]
Meaning He will be sufficient for all that is worrying him in his spiritual and worldly affairs. The one who puts his trust in Allah will have strength in his heart and will not be affected by anything he imagines or be disturbed by events, because he knows that these are the result of vulnerable human nature and of weakness and fear that have no basis. He also knows that Allah has guaranteed complete sufficiency to those who put their trust in Him. So he trusts in Allah and finds peace of mind in His promise, and thus his worry and anxiety are dispelled; hardship is turned to ease, sadness is turned to joy, fear is turned to peace.
We ask Allah to keep us safe and sound, and to bless us with strength and steadfastness of heart, and complete trust, for Allaah has guaranteed all good things to those who put their trust in Him, and has guaranteed to ward off all bad and harmful things from them.
If bad things happen or there is the fear of such, then you should count the many blessings that you are still enjoying, both spiritual and worldly, and compare them with the bad things that have happened, for when you compare them you will see the many blessings that you are enjoying, and this will make the bad things appear less serious.
See al-Wasaa’il al-Mufeedah li’l-Hayaat al-Sa’eedah by Shatkh ‘Abd al-Rahmaan ibn Sa’di
