Darukkhandhasutta 2.3

2.3

Atthi, āvuso, amumhi dārukkhandhe pathavīdhātu, yaṃ nissāya bhikkhu iddhimā cetovasippatto amuṃ dārukkhandhaṃ pathavītveva adhimucceyya.

Because the earth element exists in the tree trunk. Relying on that a mendicant with psychic powers could determine it to be nothing but earth.