That the Messenger of Allah ﷺ said: "When you see someone selling or buying in the Masjid then say: 'May Allah not profit your business.' And when you see someone announcing about something lost then say: 'May Allah not return it to you.'" The Hadith of Abu Hurairah is Hasan Gharib. This is acted upon according to some of the people of knowledge. They dislike selling and buying in the Masjid. This is the view of Ahmad and Ishaq. Some of the people of knowledge permitted selling and buying in the Masjid.  

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ إِذَا رَأَيْتُمْ مَنْ يَبِيعُ أَوْ يَبْتَاعُ فِي الْمَسْجِدِ فَقُولُوا لاَ أَرْبَحَ اللَّهُ تِجَارَتَكَ وَإِذَا رَأَيْتُمْ مَنْ يَنْشُدُ فِيهِ ضَالَّةً فَقُولُوا لاَ رَدَّ اللَّهُ عَلَيْكَ