This is what I have heard. Once the Blessed One was staying in Ayojjha, on the banks of the River Ganges. There the Blessed One addressed the monks: “It is as if, monks, this River Ganges were to carry down a large ball of foam, which a person with good eyes might see, study, and carefully scrutinize. On doing so, it would become apparent to him that it was something quite empty, quite worthless, with no substance at all. For could there be any substance in a ball of foam? In exactly the same way, monks, a monk sees, studies, and carefully scrutinizes physical form—whether past, present, or future, whether inside or outside, gross or subtle, inferior or refined, far or near. On doing so, it becomes apparent to him that it is something quite empty, quite worthless, with no substance at all. For could there be any substance in a physical form?

From Sayings of the Buddha: A Selection of Suttas from the Pali Nikayas, translated by Rupert Gethin, Oxford: Oxford University Press, 2008, p. 220.