بُلُوغُ الْمَرَامِ
Bulugh al-Maram 256
The Book of Prayer
وَعَنْهُ قَالَ : قَالَ رَسُولُ اَللَّهِ - صلى الله عليه وسلم -{ مَنْ سَمِعَ رَجُلاً يَنْشُدُ ضَالَّةً فِي اَلْمَسْجِدِ فَلْيَقُلْ : لَا رَدَّهَا اَللَّهُ عَلَيْكَ , فَإِنَّ اَلْمَسَاجِدَ لَمْ تُبْنَ لِهَذَا } رَوَاهُ مُسْلِم ٌ [1]
Narrated [Abu Hurairah (RA)]: Allah's Messenger (ﷺ) said: "Whoever hears a man crying out in the mosque about something he has lost, he should say, 'May Allah not restore it to you', for the mosques were not built for this." [Reported by Muslim].
Sunnah.com reference: Book 2, Hadith 128