Narrated Abu Hurairah: That the Messenger of Allah (ﷺ) said: "When you see someone selling or buying in the Masjid then say:

لَا أَرْبَحَ اللَّهُ تِجَارَتَكَ

laa arba'ha Allahu tijaarataka

May Allah not profit your business. This is because the mosques were not built for this purpose.

Reference: Sahih (Albani). Tirmidhi: 1321