Makhshirin, Chapter One, Mishnah Five
If one rubbed a leek or pressed his hair or his garment, Rabbi Yose says: the liquid which came out comes under the law of if water be put, but the liquid that remained does not come under the law of if water be put, because his intention was that the liquid should come out of all of it.
There are two scenarios in this mishnah. In the first he rubs off water that fell onto a leek. The second is that he squeezed out his hair or his garment to get the liquid out of them. According to Rabbi Yose in both of these cases the liquid that comes out of his hair has been separated from its source so it can now cause food to be susceptible to impurity. However, the water that remains on the leek, hair or garment does not make food susceptible to impurity because his intention was only upon the water that would come out. This fits the opinion of Bet Hillel in mishnah two.