Sunan Ibn Majah 767
سنن ابن ماجه#767SahihIt was narrated from Abu 'Abdullah, the freed slave of Shaddad bin Had that:He heard Abu Hurairah say: "I heard the Messenger of Allah say: 'Whoever hears a man making a lost-and-found announcement in the mosque, let him say: "May Allah not return it to you!" For the mosques were not built for that
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Al-AlbaniSahih
Muhammad Fouad Abd al-BaqiSahih
Shuaib Al ArnautSahih
Zubair Ali ZaiSahih Muslim
In-book reference: 4:33