—whether buying, selling, or any other pursuit—once the call to attend them is given in the mosques. Allah, exalted is He, says: “In houses that Allah has allowed to be raised and in which His name is remembered, there glorify Him morning and evening men whom neither trade nor sale distracts from the remembrance of Allah, from establishing prayer, and from paying zakah. They fear a Day when hearts and eyes will tremble, so that Allah may reward them according to the best of what they did and increase them out of His bounty; and Allah provides for whom He wills without measure” (Q 24:36–38).

Likewise, a sale is invalid if the buyer will use the item to commit disobedience to Allah or employ it in what He has forbidden. Thus it is impermissible to sell grape juice to someone who will turn it into wine, for Allah says, “And do not cooperate in sin and transgression” (Q 5:2); such a transaction is a form of assisting in wrongdoing.

By the same token, it is neither permissible nor valid to sell weapons during times of civil strife among Muslims, lest a Muslim be killed with them. All instruments of warfare fall under this ruling, for the Prophet (peace and blessings be upon him) forbade it and because of the verse, “And do not cooperate in sin and transgression.”

Ibn al-Qayyim said: “The proofs of the Sacred Law concur that the intended purpose of contracts is taken into consideration, and that this purpose determines whether a contract is valid or invalid, lawful or unlawful. If a man sells a weapon to someone he knows will use it to kill a Muslim, the sale is unlawful and void because it is aiding in sin and transgression. If, however, he sells it to someone he knows will use it to strive in the path of Allah, the sale is obedience and a meritorious act. Likewise, it is impermissible to sell weapons to those who fight the Muslims or who use them to terrorize travelers, for that too is assisting in disobedience.”