Vibhaṅgasutta

Suttapiṭaka · Saṁyuttanikāya · Mahāvaggasaṁyutta · Vol. 19 · Section 1195

[1195] And what, bhikkhus, is the mind (citta) constricted internally? Whatever mind, bhikkhus, is accompanied by dullness and drowsiness (thīna-middha), associated with dullness and drowsiness — this, bhikkhus, is called the mind constricted internally.

English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.

This passage also appears in 1 other place

Vol. 19 · Section 1182 — Vibhaṅgasutta