[3:156] O you who believe, be not as the disbelievers, who say of their brothers, when they travel in the land, or are on raiding campaigns, 'Had they been with us, they would not have died and would not have been slain' - so that God may make that anguish in their hearts. For God gives life, and He gives death and God sees what you do.
Whoever makes a habit of worrying about what is past and planning for his future and each new moment - the least of his punishment will be the constriction of his heart through disconcerting worries and the disappearance of the quality of life from his heart because of his forgetfulness and his saying 'if only such and such' or 'maybe such and such'. The result of this kind of thinking is estrangement and grief, the constriction of the heart and separation.