بُلُوغُ الْمَرَامِ
Bulugh al-Maram 257
The Book of Prayer
وَعَنْهُ : أَنَّ رَسُولَ اَللَّهِ - صلى الله عليه وسلم -{ إِذَا رَأَيْتُمْ مَنْ يَبِيعُ , أَوْ يَبْتَاعُ فِي اَلْمَسْجِدِ , فَقُولُوا : لَا أَرْبَحَ اَللَّهُ تِجَارَتَكَ } رَوَاهُ النَّسَائِيُّ , وَاَلتِّرْمِذِيُّ وَحَسَّنَهُ . [1]
Narrated [Abu Hurairah (RA)]: Allah's Messenger (ﷺ) said: "If you see someone buying or selling inside the mosque, say: 'May Allah not make your trading profitable!'" [Reported by an-Nasa'i and at-Tirmidhi and the latter graded it Hasan (good)].
Sunnah.com reference: Book 2, Hadith 129