Surah An-Nisa' (The Women), Verse 94

یٰۤاَیُّهَا الَّذِیْنَ اٰمَنُوْۤا اِذَا ضَرَبْتُمْ فِیْ سَبِیْلِ اللّٰهِ فَتَبَیَّنُوْا وَلَا تَقُوْلُوْا لِمَنْ اَلْقٰۤی اِلَیْكُمُ السَّلٰمَ لَسْتَ مُؤْمِنًا تَبْتَغُوْنَ عَرَضَ الْحَیٰوةِ الدُّنْیَا فَعِنْدَ اللّٰهِ مَغَانِمُ كَثِیْرَةٌ كَذٰلِكَ كُنْتُمْ مِّنْ قَبْلُ فَمَنَّ اللّٰهُ عَلَیْكُمْ فَتَبَیَّنُوْا اِنَّ اللّٰهَ كَانَ بِمَا تَعْمَلُوْنَ خَبِیْرًا ۟

Yaaa aiyuhal lazeena aamanoo izaa darabtum fee sabeelil laahi fatabaiyanoo wa laa taqooloo liman alqaaa ilaikumus salaama lasta mu'minan tabtaghoona 'aradal hayaatid dunyaa fa'indal laahi maghaanimu kaseerah; kazaalika kuntum min qablu famannnal laahu 'alaikum fatabaiyanoo; innallaaha kaana bimaa ta'maloona Khabeeraa

“O ye who believe! When ye go abroad in the cause of Allah, investigate carefully, and say not to any one who offers you a salutation: "Thou art none of a believer!" Coveting the perishable goods of this life: with Allah are profits and spoils abundant. Even thus were ye yourselves before, till Allah conferred on you His favours: Therefore carefully investigate. For Allah is well aware of all that ye do.”

Quran 4:94 (Yusuf Ali Translation)