Riyad as-Salihin

Abu Hurairah (May Allah be pleased with him) said: The Messenger of Allah (ﷺ) said, "When you see someone buying or selling in the mosque, say to him: 'La arbaha-Allahu tijarataka (May Allah not make your bargain profitable)!' When you see someone announcing something lost in it, say: 'May Allah not restore it to you!"'

[At-Tirmidhi].