Tricks
حَدَّثَنَا إِسْمَاعِيلُ، حَدَّثَنَا مَالِكٌ، عَنْ عَبْدِ اللَّهِ بْنِ دِينَارٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ ـ رضى الله عنهما ـ أَنَّ رَجُلاً، ذَكَرَ لِلنَّبِيِّ صلى الله عليه وسلم أَنَّهُ يُخْدَعُ فِي الْبُيُوعِ فَقَالَ " إِذَا بَايَعْتَ فَقُلْ لاَ خِلاَبَةَ ".Narrated `Abdullah bin `Umar:A man mentioned to the Prophet (ﷺ) that he had always been cheated in bargains. The Prophet (ﷺ) said, "Whenever you do bargain, say, 'No cheating
From Sahih al-Bukhari, whose reports the scholars accept as authentic.