Reference for Bava Batra 177:6
ומנין שאם אמר הריני מעיין במקום שמכריעין ולפחות לו מן הדמים והריני מכריע במקום שמעיינין ולהוסיף לו על הדמים שאין שומעין לו ת"ל אבן שלמה וצדק
And whence [may it be inferred] that we are not to listen to one who Says, 'I will level where the practice is to heap up, and reduce the price' or 'I will heap up where they level, and raise the price'? — For it has been definitely stated, A perfect and just measure thou shalt have.<span class="x" onmousemove="('comment',' Deut. XXV, 25. By deviating from the usual practice the buyer, or the seller, may be the means of defrauding, or misleading others. ');"><sup>8</sup></span> Our Rabbis taught: Whence [is it to be inferred] that the exact weight must not be given where the practice is to allow overweight, and that overweight must not be allowed where the practice is to give the exact weight? — For it has been definitely stated, <i>A perfect weight</i>.<span class="x" onmousemove="('comment',' Ibid. ');"><sup>9</sup></span>