Gopalasutta 3.2

3.2

Idha, bhikkhave, bhikkhu yaṃ kiñci rūpaṃ (…) ‘cattāri mahābhūtāni, catunnañca mahābhūtānaṃ upādāyarūpan’ti yathābhūtaṃ nappajānāti.

It’s when a mendicant doesn’t truly understand that all form is the four principal states, or form derived from the four principal states.