وَعَنِ ابْنِ عُمَرَ
قَالَ: قَالَ رَجُلٌ لِلنَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ إِنِّي أُخْدَعُ فِي الْبُيُوعِ فَقَالَ: «إِذَا بَايَعْتَ فَقُلْ: لَا خلابة «فَكَانَ الرجل يَقُوله»
A man told the Prophet (sallallahu alayhi wa sallam) that he was being deceived in business transactions, and he replied, “When you make a bargain say, ‘There is no attempt to deceive’*.” Then the man made a habit of saying that. (Bukhari and Muslim.) * He was to say this to protect himself because he was not expert enough to detect defects in what he was buying.
Some hold that one who says this has the right to cancel the transaction if any defect is discovered within three days. Some say this applied only to the man who spoke to the Prophet (sallallahu alayhi wa sallam); others say it applies to anyone.