Godattasutta

Suttapiṭaka · Saṁyuttanikāya · Saḷāyatanavaggasaṁyutta · Vol. 18 · Section 574

[574] "And what, venerable sir, is the emptiness liberation of mind (suññatā cetovimutti)? Here, venerable sir, a bhikkhu, gone to the forest, or gone to the root of a tree, or gone to an empty dwelling, reflects thus: 'This is empty of a self or of what belongs to a self.' This, venerable sir, is called the emptiness liberation of mind."

English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.