Sahih Muslim Book 10, Hadith Number 3832.

Chapter : Conversion of currency and selling of gold for silver on the spot.

Humaid reported Anas (Allah be pleased with him) having said this: Allah’s Apostle (may peace be upon him) called for young cupper belonging to us. He capped him and he (the Holy Prophet) commanded that he should be paid one sa’ or one mudd or two mudds (of wheat). It was said (that charges were high) and a reduction was made in the charges.