So I have heard. At one time the Buddha was staying near Sāvatthī in Jeta’s Grove, Anāthapiṇḍika’s monastery.

Then the Buddha robed up in the morning and, taking his bowl and robe, entered Sāvatthī for alms. And Venerable Rāhula also robed up and followed behind the Buddha.

Then the Buddha looked back at Rāhula and said, “Rāhula, you should truly see any kind of form at all—past, future, or present; internal or external; solid or subtle; inferior or superior; far or near: <em>all</em> form—with right understanding: ‘This is not mine, I am not this, this is not my self.’” “Only form, Blessed One? Only form, Holy One?” “Form, Rāhula, as well as feeling and perception and choices and consciousness.”

Then Rāhula thought, “Who would go to the village for alms today after being advised directly by the Buddha?” Turning back, he sat down cross-legged at the root of a certain tree, setting his body straight, and bringing mindfulness to the present.

Venerable Sāriputta saw him sitting there, and addressed him, “Rāhula, develop mindfulness of breathing. When mindfulness of breathing is developed and cultivated it’s very fruitful and beneficial.”

Then in the late afternoon, Rāhula came out of retreat, went to the Buddha, bowed, sat down to one side, and said to him:

“Sir, how is mindfulness of breathing developed and cultivated to be very fruitful and beneficial?”

“Rāhula, the interior earth element is anything internal, personal, that’s hard, solid, and appropriated. This includes: head hair, body hair, nails, teeth, skin, flesh, sinews, bones, bone marrow, kidneys, heart, liver, diaphragm, spleen, lungs, intestines, mesentery, undigested food, feces; or anything else internal, personal, that’s hard, solid, and appropriated. This is called the interior earth element. The interior earth element and the exterior earth element are just the earth element. This should be truly seen with right understanding like this: ‘This is not mine, I am not this, this is not my self.’ When you truly see with right understanding, you grow disillusioned with the earth element, detaching the mind from the earth element.