Sahih Bukhari Volume 3, Book 42, Hadith Number 619.
Narated By Abu Bakr : While I was on my way, all of a sudden I saw a shepherd driving his sheep, I asked him whose servant he was. He replied that he was the servant of a man from Quraish, and then he mentioned his name and I recognized him. I asked, “Do your sheep have some milk?” He replied in the affirmative. I said, “Are you going to milk for me?” He replied in the affirmative. I ordered him and he tied the legs of one of the sheep. Then I told him to clean the udder (teats) of dust and to remove dust off his hands. He removed the dust off his hands by clapping his hands. He then milked a little milk. I put the milk for Allah’s Apostle in a pot and closed its mouth with a piece of cloth and poured water over it till it became cold. I took it to the Prophet and said, “Drink, O Allah’s Apostle!” He drank it till I was pleased.