This was said by the Blessed One, said by the Arahant, as I have heard:
“Bhikkhus, for a bhikkhu practicing in accordance with the |Dhamma::teachings of the Buddha that point to the nature of reality, the ultimate truth [dhamma]|, this is the natural way of expressing himself: when speaking of being such a practitioner, he speaks only in terms of the Dhamma, not in terms of what is not the Dhamma. When thinking, he thinks only Dhamma-aligned thoughts, not thoughts contrary to the Dhamma. Or, having transcended both, he abides with |equanimity::mentally poised, mentally balanced, equanimous, non-reactive, looking on [upekkhaka]|, |mindful::remembering to be present with continuous effort, observing the body in and of itself, feelings in and of itself, mind in and of itself, mental qualities in and of itself. Read [SN 47.2 Sati sutta - Mindfulness](/sn47.2) to learn more [sata]| and |fully aware::with attentiveness, with clear and full comprehension, intentional, purposeful [sampajāna]|.”
The Blessed One spoke on this matter. In this regard, it is said:
“A bhikkhu delighting in Dhamma,
reflecting upon the Dhamma,
recollecting the Dhamma,
does not fall away from the true Dhamma.
Whether walking or standing,
sitting or lying down—
with the mind settled inwardly,
he attains only |peace::calm, tranquility [santi]|.”
This matter too was spoken by the Blessed One, as I have heard.