Sunan an-Nasa'i 4484

أَخْبَرَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، عَنْ مَالِكٍ، عَنْ عَبْدِ اللَّهِ بْنِ دِينَارٍ، عَنِ ابْنِ عُمَرَ، أَنَّ رَجُلاً، ذَكَرَ لِرَسُولِ اللَّهِ صلى الله عليه وسلم أَنَّهُ يُخْدَعُ فِي الْبَيْعِ فَقَالَ لَهُ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ إِذَا بِعْتَ فَقُلْ لاَ خِلاَبَةَ ‏"‏ ‏.‏ فَكَانَ الرَّجُلُ إِذَا بَاعَ يَقُولُ لاَ خِلاَبَةَ ‏.‏

It was narrated from Ibn 'Umar that:a man told the Messenger of Allah that he was always being cheated. The Messenger of Allah said to him: "When you make a deal, say: There is no intention of cheating" So, whenever the man engages in a deal he says, 'There is no intention of cheating." "(Sahih)

Reference

Sunan an-Nasa'i 4484

In-book

Book 44, Hadith 36