Musnad Ahmad 30:1467

Abu Abd al-Rahman al-Muqri narrated to us, Haywah narrated to us, he said: I heard Abu al-Aswad saying: Abu Abd Allah, the freed slave of Shaddad, informed me 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 to him: 'May Allah not return it to you,' for the mosques were not built for this."