Vibhaṅgasutta

Suttapiṭaka · Saṁyuttanikāya · Mahāvaggasaṁyutta · Vol. 19 · Section 861

[861] And what, bhikkhus, is the faculty of mindfulness (satindriya)? Here, bhikkhus, a noble disciple (ariyasāvaka) is mindful, possessed of supreme mindfulness and discretion (paramena satinepakkena), one who remembers and recollects what was done long ago and what was said long ago. This, bhikkhus, is called the faculty of mindfulness.

English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.

This passage also appears in 1 other place

Vol. 19 · Section 873 — Mudutavagga