Anushasana Parva 140.93

The froth of that water is highly sacred and it partook of His own nature. Those Rishis who live upon the measure of froth that thus came out are called. Phenapas (Frotheaters). This is the conduct of those pure Rishis, O lady having penances for wealth! Listen now to me as I explain to you who the Valakhilyas arc.