Majjhima Nikaya 2:156
SUJATOSantaṁ vā ajjhattaṁ upekkhāsambojjhaṅgaṁ ‘atthi me ajjhattaṁ upekkhāsambojjhaṅgo’ti pajānāti, asantaṁ vā ajjhattaṁ upekkhāsambojjhaṅgaṁ ‘natthi me ajjhattaṁ upekkhāsambojjhaṅgo’ti pajānāti, yathā ca anuppannassa upekkhāsambojjhaṅgassa uppādo hoti tañca pajānāti, yathā ca uppannassa upekkhāsambojjhaṅgassa bhāvanāya pāripūrī hoti tañca pajānāti.
equanimity in them, they understand: ‘I have the awakening factor of equanimity in me.’ When they don’t have the awakening factor of equanimity in them, they understand: ‘I don’t have the awakening factor of equanimity in me.’ They understand how the awakening factor of equanimity that has not arisen comes to arise; and how the awakening factor of equanimity that has arisen becomes fulfilled by development.