Sahih Muslim
The Book of Transactions (Kitab Al-Buyu)
Book Number: 010 / Hadith Number: 3834
Ibn 'Abbas (Allah be pleased with them) reported: The slave of Banu Bayada cupped Allah's Apostle (may peace be upon him) and he gave him his wages, and talked to his master and he reduced the charges, and if this earning was unlawful Allah's Apostle (may peace be upon him) would not have given it.