Translation:

156. O ye who believe! be not like the unbelievers who say of their brethren when they are travelling through the earth or engaged in fighting: "If they had stayed with us they would not have died or been slain." This that Allah may make it a cause of sighs and regrets in their hearts. It is Allah that gives life and death and Allah sees well all that ye do.

Notes (Tafseer)

469. It is want of faith that makes people afraid (1) of meeting death, (2) of doing their duty when it involves danger, as in travelling in order to earn an honest living, or fighting in a sacred cause. Such fear is part of the punishment for want of faith. If you have faith, there is no fear in meeting death, for it brings you nearer to your goal, nor in meeting danger for a sufficient cause, because you know that the keys of life and death are in Allah's hands. Nothing can happen without Allah's Will. If it is Allah's Will that you should die, your staying at home will not save you. If it is His Will that you should live, the danger you incur in a just cause brings you glory. Supposing it is His Will that you should lose your life in the danger, there are three considerations that would make you eager to meet it: (1) dying in doing your duty is the best means of reaching Allah's Mercy; (2) the man of faith knows that he is not going to an unknown country of which he has no news; he is going nearer to Allah; and (3) he is being "brought together" unto Allah; i.e., he will meet all his dear ones in faith: instead of the separation which the souls without faith fear, he looks forward to a surer reunion than is possible in this life.