the Holy Qur'an

O ye who believe! When ye go abroad in the cause of God, 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 God are profits and spoils abundant. Even thus were ye yourselves before, till God conferred on you His favours: Therefore carefully investigate. For God is well aware of all that ye do. (4:94)

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