Is Announcing Lost Items in the Mosque Allowed?
Question

In the Hadith, it says that the Prophet (blessings and peace of Allah be upon him) said: “Whoever hears a man making a lost property announcement in the mosque, let him say: May Allah not restore it to you, for the mosques were not built for this purpose.”
In the light of this Hadith, if someone loses something valuable in the mosque and does not announce it – rather he tells the Imam or the caretaker – do they come under the prohibition that is mentioned in the Hadith? 
Does the prohibition that is mentioned in the Hadith apply only to one who loses something, or is it general in meaning, applying to making any kind of announcement in the mosque?
Praise be to Allah.Is it allowed to announce lost items in the mosque?
It is not permissible to make lost property announcements in the mosques or to make announcements about items that have been found, because the mosques were not built for that; rather the mosques were built for the remembrance of Allah.
Muslim (568) narrated that Abu Hurayrah (may Allah be pleased with him) said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “Whoever hears a man making a lost property announcement in the mosque, let him say: `May Allah not restore it to you, for the mosques were not built for this purpose.’”
Muslim (569) also narrated from Buraydah (may Allah be pleased with him) that a man made a lost property announcement in the mosque, saying: Has anyone found the red camel? The Prophet (blessings and peace of Allah be upon him) said: “May you not find it. The mosques were only built for that for which they were built.”
An-Nawawi (may Allah have mercy on him) said:
“The words of the Prophet (blessings and peace of Allah be upon him), “May you not find it,” and his instruction to say such words are a punishment [to the one who makes such announcements] for going against his teachings and disobeying him. The one who hears him [making such an announcement] should say: “May you not find it, for the mosques were not built for this.” Or he may say: “May you not find it; the mosques were only built for that for which they were built” – as the Messenger of Allah (blessings and peace of Allah be upon him) said.”
Ibn `Abd Al-Barr (may Allah have mercy on him) said:
“Allah, may He be Exalted, has stated that the mosques are houses which Allah has ordered to be raised and that His name be mentioned therein; exalting Him within them in the morning and the evening [An-Nur 24:36]. It is for this purpose that they were built, so they should be protected from everything for which they were not built." (Al-Istidhkar 2/368)
Al-Bayhaqi (may Allah have mercy on him) narrated in As-Sunan (20763) that `Umar ibn Al-Khattab (may Allah be pleased with him) built a public courtyard next to the mosque and called it Al-Butayha’. He used to say: Whoever wants to make noise or recite poetry or raise his voice, let him go out to this public courtyard. (See: Al-Istidhkar, 2/368)
So whoever wants to make a lost property announcement, let him go outside the mosque, and if he writes on a piece of paper and sticks it on the outside wall of the mosque, there is nothing wrong with that.
Whoever loses something in the mosque, and wants to tell the Imam of the mosque or the Mu’adhdhin or the caretaker, so that they can tell the one who finds it who it belongs to, and the like, there is nothing wrong with that, if he speaks to him privately.
Shaykh Ibn `Uthaymin (may Allah have mercy on him) said:
“The mosques were not built for the making of lost property announcements or buying and selling; rather they were built for the worship of Allah, for prayer, Dhikr, circles of knowledge and the like.
As for writing announcements on pieces of paper and posting them in the mosque, if that is on the outside wall, there is nothing wrong with it, or if it is on an outer door, there is nothing wrong with it. As for posting them inside, that is not appropriate, because it is akin to speaking, and because it may distract people who look at the notice and read it.
What appears to me that it is not permissible, because posting notices in the mosque is the same as making lost property announcements. But if it is written on the outside wall, or it is written on the door and is outside the mosque, there is nothing wrong with that." (Fatawa Nur `Ala Ad-Darb,  2/709)
Shaykh Ibn `Uthaymin (may Allah have mercy on him) said:
“As Allah has stated that the mosques are His houses, and the Prophet (blessings and peace of Allah be upon him) described them in the same way, and they are houses which Allah has ordered to be raised, they have sanctity, there are rulings which apply to them, and they should be respected.
One of the rulings regarding mosques is that it is not permissible to buy and sell in them, whether on a small scale or a large scale. Similarly, it is not permissible to make lost property announcements in them, when a man comes and says: I have lost such and such, such as a wallet. This is not permissible even if you think it likely that it was stolen in the mosque; you should not say, How can I get it back? Go and sit at the door of the mosque, outside the mosque, and say: May Allah reward you, I have lost such and such. The point is that the mosques must be respected. (Sharh Riyadh As-Salihin  p. 2014).
The scholars of the Permanent Committee said:
“He can post a notice outside the door of the mosque, in a specific spot, so that people can read it." (Fatawa Al-Lajnah Ad-Da’imah,  5/275)
What can you announce in the mosque?
The prohibition is general in meaning and applies to all announcements made in the mosque, except announcements having to do with acts of worship – there is nothing wrong with that.
The scholars of the Permanent Committee said:
“It is not permissible to take the mosques, or any of their facilities, or the external courtyards and precincts that belong to them, as places to set up noticeboards, advertisements and other commercial signage, whether that is for schools, factories, companies or otherwise. That is because the mosques have only been built for the worship of Allah, may He be Exalted, through prayer, Dhikr, learning and teaching Islamic knowledge, reciting Quran and other religious matters.
So they should be protected from the things mentioned, and their sanctity should be preserved. We should be keen not to distract people with things that will divert them from worshipping Allah, may He be Exalted, and focusing on the hereafter." (Fatawa Al-Lajnah Ad-Da’imah, 5/276-277)
Shaykh Ibn `Uthaymin (may Allah have mercy on him) was asked:
An announcement was made in one of the mosques that Iftar would be provided for everyone who wanted to fast every Thursday. What is the ruling on that?
He replied: 
“There is nothing wrong with announcing Iftar meals , because it is an announcement which calls people to something good, and the purpose behind it is not buying or selling. What is prohibited is making announcements about buying and selling, or renting and hiring, which are things for which the mosques were not built. As for calling people to something good, feeding people and giving charity, there is nothing wrong with that.”
He (may Allah have mercy on him) was also asked: What is the ruling on posting some advertisement in the mosque, such as an advertisement about a trip for Hajj or `Umrah, or advertisements about lectures and lessons?
He (may Allah have mercy on him) replied: 
“If the advertisement has to do with an act of worship, there is nothing wrong with it, because acts of worship are things that draw people closer to Allah, and the mosques were built for the worship of Allah, may He be Glorified and Exalted.
But if the advertisement has to do with worldly matters , then it is prohibited. Rather it may be advertised via posters on the outside wall of the mosque. Hajj group trips are a worldly matter, so we do not think that they should be advertised inside the mosque.
Circles of Dhikr – such as courses and classes on Islamic knowledge – are pure good, so there is nothing wrong with announcing them inside the mosque, because they are something good." (Sharh Manzumat Al-Qawa`id Al-Fiqhiyyah  p. 52)
For more, see these answers:
Advertising a campaign for vaccination of children in the mosque
Making death announcements from mosque minarets  
And Allah knows best.