Dasuttarasutta

Suttapiṭaka · Dīghanikāya · Pāthikavagga · Vol. 11 · Section 373

[373] What one thing is hard to penetrate? The concentration of mind (cetosamādhi) with immediate result (anantarika). This is the one thing hard to penetrate.

English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.