[242] On one occasion the Blessed One was dwelling at Ayujjhā, on the bank of the river Gaṅgā. There the Blessed One addressed the bhikkhus: "Bhikkhus, suppose this river Gaṅgā were to carry along a great lump of foam. A man with eyes would see it, ponder it, and methodically examine it; and as he sees it, ponders it, and methodically examines it, it would appear to him as merely void, it would appear as merely hollow, it would appear as merely without substance (sāra). For what substance, bhikkhus, could there be in a lump of foam? In just the same way, bhikkhus, whatever form there is—past, future, or present … whether far or near—a bhikkhu sees it, ponders it, and methodically examines it; and as he sees it, ponders it, and methodically examines it, it appears to him as merely void, it appears as merely hollow, it appears as merely without substance. For what substance, bhikkhus, could there be in form?"
English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.