And from him (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "If you see someone selling or buying in the mosque, say to him:

The Shaykh : All right, let us begin today's lesson. He said: "And from him—meaning Abu Hurayrah—(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

إذا رأيتم من يبيع أو يبتاع

Jami` at-Tirmidhi 1321Sahih (Darussalam)

" The word "you see" could mean that the intended meaning of "seeing" is knowledge, and it could also mean seeing with the eye. These two possibilities do not contradict each other, because the one who sees the speaker with his eye, what has he done?
The student: Knowledge.

The Shaykh : So he has knowledge. And the one who is blind but heard has knowledge. Accordingly, if we want to make it more general, we say that the intended meaning of "seeing" here is the seeing of knowledge.
The difference is that the seller is the one from whom the commodity is sought, and the buyer is the one who sought the commodity. This definition is more general than saying the seller is the one who sold the goods and the buyer is the one who offered the money, because sometimes the sale itself is what is sought. Therefore, we say the difference between them is that the seller's commodity is sought, and the buyer is, yes, the seeker. Is that clear?
The student: It is clear.

The Shaykh : All right, if he sold a garment for a turban, which one is the item being sold?
The student: The garment.

The Shaykh : The garment. All right, if he sold a garment for a dinar?
The student: The garment.

The Shaykh : The garment. If he sold a dinar for a garment?
The student: The dinar.

The Shaykh : No, if he sold a dinar for a garment.
The student: The dinar.

The Shaykh : How? We ask: which one is stronger now, is the dinar the one being sought or the garment?
The student: The dinar.

The Shaykh : The dinar, because it is the item being sold. And what is known among the jurists is that whatever the preposition "ba" (for) enters upon is the price, because "ba" indicates exchange and substitution. So whatever "ba" enters upon is the price. And the one who offers the price is who?
The student: The buyer.

The Shaykh : The buyer, whether what "ba" entered upon is money or goods. Accordingly, if I say, "I sold you a garment for a dinar," what is the price?
The student: The dinar.

The Shaykh : The dinar. And if I say, "I sold you a dinar for a garment," the price is the garment. Done.
"Say to him"—the command is directed to everyone. Is it required of every individual, or is the intention the collective without the aggregate? The second is what is intended; the second is what is intended. The first meaning is possible. So according to the second, if one person among the people says it, that suffices. And according to the first, it is necessary that everyone who heard it say it.
And which is more effective in deterrence? That it is for everyone; that it is for everyone. Meaning, may He not make there any profit. And trade is the wealth in which profit is sought, of whatever type it may be: clothes, vessels, wood, iron, cars, machines, or others. Everything in which profit is sought is trade. Therefore, we curse him with what contradicts his intention, because he only sold and bought in the mosque with the intention of profit. So we curse him with what contradicts his intention: May Allah not make your trade profitable.
"It is said: It was narrated by al-Nasa'i and al-Tirmidhi, and he graded it as good". And it is said in explaining the reason for this what we said in explaining the reason for calling out a lost item: namely, that mosques were not built for this, i.e., for selling and buying, but rather they were built for the remembrance of Allah the Exalted, reciting the Qur'an, prayer, and what is similar to that.