ياأيها الذين آمنوا إذا ضربتم في سبيل الله فتبينوا ولا تقولوا لمن (4:94) بالانجليزي

(4:94) Believers! When you go forth in the way of Allah, discern (between friend and foe), and do not say to him who offers you the greeting of peace: 'You are not a believer.' *126 If you seek the good of this worldly life, there lies with Allah abundant gain. After all, you too were such before, and then Allah was gracious to you. *127 Discern, then, for Allah is well aware of what you do.

Tafseer Abu-al-A'la Maududi

Page 93 English Traditional

*126).In the early days of Islam the greeting as-salam 'alaykum ('peace be on you') was a distinguishing symbol of the Muslims. When a Muslim greeted another Muslim with this expression it signified that he was a member of the same community, that he was a friend and well-wisher, one who wished peace and security, from whom he need entertain no fear of hostility and towards whom, in return, he should not behave with hostility. The Islamic greeting occupied virtually the same position among Muslims as the passwords used by sentries to distinguish friend from foe. This was particularly important in those days because there were no distinctions in dress, language and so on by which Muslims could be conclusively marked off from their non-Muslim Arab compatriots.
The Muslims also encountered a strange problem on the battlefield. Whenever a Muslim was in danger of being harmed inadvertently by other Muslims during the fighting, he resorted to either the Islamic greeting (as-salam 'alaykum) or the Islamic creed There is no god save Allah' (ili-Jl^y) in order to indicate that he was their brother-in-faith. The Muslims, however, often suspected this to be merely a ruse of the enemy and therefore sometimes disregarded the utterance of the Islamic greeting or of the Islamic creed, and killed such people and seized their belongings as booty. Although whenever the Prophet (peace be on him) came to know of such incidents, he severely reproached the people concerned, it, nevertheless, continued to take place. In the end God solved the problem by revelation. The purport of the verse is that no one has the right summarily to judge those who profess to be Muslims, and assume them to be lying for fear of their lives. At least two possibilities exist: the claim may either be true or it may be false. The truth can only be ascertained by proper investigation. While it is impossible to investigate a person's case properly during fighting and this may enable him to save his life by lying, it is equally possible that an innocent, true believer might be put to death by mistake. The error of letting an unbeliever go unpunished is preferable to that of killing a true believer.
*127).The Muslims are now told that there was a time when they were scattered among different tribes of unbelievers. They were, therefore, forced to conceal the fact of being Muslims since they feared that they would be subjected to persecution and hardship. In those days they had nothing else besides their verbal profession to testify to their faith. Later on, some time before these verses were revealed, God benevolently enabled the Muslims to develop a collective entity of their own and thus to raise the banner of Islam in the face of strong opposition from the unbelievers. That the Muslims should fail to appreciate the hardships which other Muslims were enduring, and which they themselves had endured until not long before, and not to treat them with consideration and forbearance, did not seem an adequate way of thanking God for His benevolence.