Majjhima Nikaya 2:16
SUJATOIdha, bhikkhave, bhikkhu araññagato vā rukkhamūlagato vā suññāgāragato vā nisīdati, pallaṅkaṁ ābhujitvā, ujuṁ kāyaṁ paṇidhāya, parimukhaṁ satiṁ upaṭṭhapetvā.
It’s when a mendicant—gone to a wilderness, or to the root of a tree, or to an empty hut—sits down cross-legged, sets their body straight, and establishes mindfulness in their presence.