Brahmajalasutta 1.31.1

1.31.1

Idha, bhikkhave, ekacco samaṇo vā brāhmaṇo vā ātappamanvāya padhānamanvāya anuyogamanvāya appamādamanvāya sammāmanasikāramanvāya tathārūpaṃ cetosamādhiṃ phusati, yathāsamāhite citte (…) anekavihitaṃ pubbenivāsaṃ anussarati.

It’s when some ascetic or brahmin—by dint of keen, resolute, committed, and diligent effort, and right application of mind—experiences an immersion of the heart of such a kind that they recollect their many kinds of past lives.