Musnad Ahmad 30:2331

Harun ibn Ma'ruf narrated to us, he said: Ibn Wahb narrated to us, Haywah informed me, from Muhammad ibn Abd al-Rahman, from Abu Abd Allah, the freed slave of Shaddad ibn al-Had, that he heard Abu Hurayrah saying: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "Whoever hears a man searching for a lost item in the mosque, let him say: 'May Allah not return it to you,' for the mosques were not built for that."