Almighty Allah said: So do not become weak (against your enemy), nor be sad, and you will be superior (in victory) if you are indeed (true) believers. If a wound (and killing) has touched you, be sure a similar wound (and killing) has touched the others. And so are the days (good and not so good), We give to men by turns, that Allah may test those who believe, and that He may take martyrs from among you. And Allah likes not the Zalimeen (polytheists and wrongdoers).

And that Allah may test (or purify) the believers (from sins) and destroy the disbelievers. Do you think that you will enter Paradise before Allah tests those of you who fought (in His Cause) and (also) tests those who are patient? You did indeed wish for death (Ash-shahadah - martyrdom) before you met it. Now you have seen it openly with your own eyes. Surah 3: 139-143

Allah's Message to the Cowards - Qur'anic
Allah the Exalted also said: We shall cast terror into the hearts of those who disbelieve, because they joined others in worship with Allah for which He had sent no authority their abode will be the Fire and how evil is the abode of the Zalimeen (polytheists and wrong-doers). And Allah did indeed fulfill His Promise to you when you were killing them (your enemy) with His Permission; until (the moment) you lost your courage and fell to disputing about the order, and disobeyed after He showed you (of the booty) which you love. Among you are some that desire this world and some that desire the Hereafter. Then He made you flee from them (your enemy), that He might test you. But surely He forgave you, and Allah is most Gracious to the believers.

(And remember) when you ran away (dreadfully) without even casting a side glance at anyone, and the Messenger (Muhammad) was in your rear calling you back. There did Allah give you one distress after another by way of requital to teach you not to grieve for that which had escaped you, nor for that which had befallen you. And Allah is Well-Aware of all that you do.

Then after the distress, He sent down security for you. Slumber overtook a party of you, while another party was thinking about themselves (as how to save their ownselves, ignoring the others and the Prophet) and thought wrongly of Allah - the thought of ignorance. They said, "Have we any part in the affair?" Say you (0 Muhammad): "Indeed the affair belongs wholly to Allah." They hide within themselves what they dare not reveal to you, saying: "If we had anything to do with the affair none of us would have been killed here." Say: "Even if you had remained in your homes, those for whom death was decreed would certainly have gone forth to the place of their death," but that Allah might test what is in your breasts; and to Mahis to test, to purify to get rid of that which was in your hearts (sins), and Allah is All-Knower of what is in (your) breasts. " Surah 3: 151-154