Hadith: Whoever hears a man inquiring in the mosque about something he has lost should say: "May Allah not restore it to you," for indeed mosques are not built for this

عن أبي هريرة رضي الله عنه قال: قال رسول الله صلى الله عليه وسلم : «من سمِع رجُلا يَنْشُدُ ضَالَّةً في المسجد فليقُل: لا رَدَّهَا الله عليك، فإنَّ المساجد لم تُبْنَ لهذا».
[صحيح] - [رواه مسلم]

Abu Hurayrah (may Allah be pleased with him) reported that the Messenger of Allah (may Allah's peace and blessings be upon him) said: "Whoever hears a man inquiring in the mosque about something he has lost should say: 'May Allah not restore it to you,' for indeed mosques are not built for this."
Sahih/Authentic. - [Muslim]

Explanation

This Hadīth teaches us that if we find someone asking in the mosque about something he has lost, we should say to him: "May Allah not restore it to you" or "May you not find it", according to another narration. This aims at scolding such a person for his lack of reverence for the mosque. Then, the Prophet (may Allah's peace and blessings be upon him) clarified the reason for this rebuke, saying: "Indeed, mosques were not built for this." In fact, mosques are built for remembering Allah, the Almighty; prayer; knowledge; good discussions; and so on. Therefore, as the seeker of a lost item in the mosque has used this sacred place for a purpose not befitting it, he deserves supplication against him that he may not find his lost property. This is to punish him by giving him the opposite of what he seeks and also to intimidate others and repel them from doing the same. Overall, the Hadīth falls under commanding the right and forbidding the wrong, with the relevant conditions. If the supplication deters such a person and makes him stop, that is good; otherwise, it should be repeated.