Mudutavagga

Suttapiṭaka · Saṁyuttanikāya · Mahāvaggasaṁyutta · Vol. 19 · Section 872

[872] And what, bhikkhus, is the faculty of energy (viriyindriyaṃ)? Bhikkhus, the energy that one obtains in dependence on the four right strivings (cattāro sammappadhāne) — this, bhikkhus, is called the faculty of energy.

English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.

This passage also appears in 1 other place

Vol. 19 · Section 860 — Vibhaṅgasutta