Sayyiduna Abu Hurayrah (may Allah be pleased with him) said: I heard the Messenger of Allah (peace and blessings be upon him) say: "Whoever hears a man announcing a lost item in the mosque, let him say: 'La addaha Allahu ilayk' (May Allah not return it to you), for the mosques were not built for this."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب اللقطة / 12104
Ibn Buraydah (may Allah have mercy on him) narrates from his father (Sayyiduna Buraydah, may Allah be pleased with him) that the Prophet (peace and blessings be upon him) heard a Bedouin, or some man, saying: Who will bring the red camel? The Messenger of Allah (peace and blessings be upon him) said: "May you not find it! These mosques are only for the purpose for which they were built," meaning for worship.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب اللقطة / 12105