“If you see someone buying or selling in the mosque, say: ‘May Allah not bless your trade.’ And if you see someone seeking a lost item therein, say: ‘May Allah not return it to you.’”

Narrated by at-Tirmidhī (no. 1321), an-Nasā’ī in al-Kubrā (no. 9933), al-Dārimī (no. 1441), Ibn Khuzaymah (no. 1305), and al-Ḥākim (no. 2339), from the narration of Abū Hurayrah (may Allah be pleased with him).
Authenticated in Ṣaḥīḥ al-Jāmiʿ (no. 573), and Ṣaḥīḥ at-Targhīb wa at-Tarhīb (no. 291).

Brief Explanation of the Hadith

The mosques are the houses of Allah, established for worship, remembrance of Allah, the performance of prayer, and other acts pertaining to the Hereafter. They are meant to direct the hearts toward the Hereafter and away from worldly concerns. To uphold this purpose, the Prophet (may Allah’s peace and blessings be upon him) instructed that the mosques be safeguarded from worldly matters such as buying and selling, announcing lost items, and other similar activities. Hence, the Prophet (may Allah’s peace and blessings be upon him) said: “If you see someone selling or buying in the mosque” — meaning, if you observe someone offering goods for sale or purchasing them within the mosque, or if you come to know of it — “then say to him openly and aloud, whether collectively or individually, as it is a communal obligation:
‘May Allah not grant profit to your trade.’”
You are to supplicate against him with loss and deprivation, so that his trade becomes stagnant and brings no profit — a form of punishment that opposes his intent, and a deterrent from such conduct. This serves as a stern warning to prevent mosques from turning into marketplaces for buying and selling, which would compromise their sanctity and strip them of the tranquility and reverence for which they were built. It would also disturb the peace and quiet required for worshippers and those in retreat.
This contains evidence for the prohibition of buying and selling in the mosques, whether the transaction is minor or major. Some scholars held that it is disliked (makrūh tanzīhi). Others stated that a sale conducted in the mosque is invalid, though the more correct view is that the contract is not nullified. However, other than buying and selling, other types of transactions are permissible in the mosque, such as giving gifts, forgiving debts, conducting marriage contracts, repaying or collecting debts, offering loans, and the like.
Part of preserving the sanctity of the mosques is that if you see someone announcing a lost item in the mosque—whether you see him doing so or learn of it—meaning that he raises his voice asking about a lost possession, whether it be an animal or otherwise (though some scholars specify it as an animal)—then say to him: “May Allah not return it to you.”
This is a supplication against him, that he may not recover what he lost, as a means of rebuke for failing to show proper reverence to the mosque, and for raising his voice and causing disturbance to those who are praying or in retreat therein.
The ḥadīth highlights the sanctity of the mosques and the obligation to honor them. So, if this stern warning applies to one who engages in trade—even though it is a lawful source of income—within the mosque, then how much more severe is the case of one who turns the mosque into a place for idle talk and false speech?