Abu Hurairah (May Allah be pleased with him) said:
The Messenger of Allah 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]. (DE)
App reference (DE): Book 18, Hadith 187 (DE)
Arabic/English book reference (DE): Book 18, Hadith 1697 (DE)