And from him, he said: The Messenger of Allah (peace and blessings be upon him) said: “Whoever hears a man calling out for a lost item in the mosque, let him say:

The Shaykh :And from him (may Allah be pleased with him), he said: The Messenger of Allah (peace and blessings be upon him) said:

If anyone hears a man crying out in the mosque about something he has lost, he should say: May Allah not restore it to you, for the mosques were not built for this.

من سمع رجلاً ينشد ضالة في المسجد فليقل لا ردها الله عليك فإن المساجد لم تبن لهذا

Sahih Muslim 568a

. Muslim narrated it.
And from him (may Allah be pleased with him), that the Messenger of Allah (peace and blessings be upon him) said:

When you see someone selling or buying in the Masjid then say: 'May Allah not profit your business.'

إذا رأيتم من يبيع أو يبتاع في المسجد فقولوا له لا أربح الله تجارتك

Jami` at-Tirmidhi 1321Sahih (Darussalam)

. Al-Nasa'i and al-Tirmidhi narrated it, and he graded it as good. ".
His statement: The word “man” is based on the common case, and even if it were assumed that he heard a woman, the ruling is the same. “Calling out for a lost item” means asking about it: Who saw the lost item? Who identified it? Who kept it? And similar expressions.
And the lost item is the stray livestock, namely the lost camel, the lost sheep, and the lost cow. So whoever hears someone calling out for a lost item, let him say this: The phrase here is declarative because the verb in it is a negated past tense, and the intended meaning is supplication. The intended meaning is supplication, meaning that when you hear him, you supplicate to Allah that He not return it to him.
It is possible that this phrase is an explanation by the Prophet (peace and blessings be upon him and his family) for the ruling. It is possible that it is coupled with the statement to this caller, meaning that we say when we hear someone calling out for a lost item: “May Allah not return it to you, for the mosques were not built for this.” It is possible that we limit ourselves to saying “May Allah not return it to you” and say that the Prophet (peace and blessings be upon him) explained the supplication for not returning it by saying that the mosques were not built for this. The possibilities are balanced, but in this case, one looks at what the situation requires. If saying it to the one calling out provides him with reassurance, then it is better to say it. And if it might open the door to argument, where he says, “The mosques were not built for this, but what is the objection?” then it is better to omit it. A person looks at the benefit in this. If the benefit is not clear, then it is better to say it, because it will undoubtedly convince this caller if it is made clear to him that the mosques were not built for this. So for what were they built? They were built, as the Prophet (peace and blessings be upon him) said, for reciting the Qur'an, dhikr, prayer, knowledge, and the like.