Satipaṭṭhānavibhaṅgasuttantabhājanīya

Abhidhammapiṭaka · Vibhaṅga · Vol. 35 · Section 454

[454] "Ardent (ātāpī)": Herein, what is ardour (ātappa)? That which is the mental (cetasika) arousing of energy (viriyārambha) … right effort (sammāvāyāma) — this is called ardour. He is endowed with this ardour … possessed of it: therefore he is called "ardent."

English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.

This passage also appears in 1 other place

Vol. 35 · Section 437 — Satipaṭṭhānavibhaṅgasuttantabhājanīya