This was said by the Blessed One, said by the Arahant, thus have I heard:

“Bhikkhus, a bhikkhu endowed with two qualities lives contented in this very life, without distress, without trouble, without fever; and upon the breaking up of the body, after death, a good destination is expected. What two? Guarding of the sense doors and moderation in eating. Endowed with these two qualities, bhikkhus, a bhikkhu lives contented in this very life, without distress, without trouble, without fever; and upon the breaking up of the body, after death, a good destination is expected.”

The Blessed One spoke on this matter. Therefore it is said thus:

"The eye and the ear, and the nose, the tongue, the body, and likewise the mind; for a bhikkhu who has these doors, well-guarded here —

Moderate in eating, restrained in the senses; He gets to bodily and mental ease, and experiences contentment.

With a body not burning, with a mind not burning; Whether by day or by night, such a one dwells in contentment."

This matter too was spoken by the Blessed One, thus have I heard.

One can independently verify this teaching by applying it for a period of several weeks and observing for the subsiding of the restlessness/agitation/anxiety in the mind.

Related Teachings:

Diverse Perceptions, Intentions, Passions, Quests (SN 14.7) arise from a lack of sense restraint and mindfulness,

The end of the world can’t be reached through experiences (AN 4.45) - In this teaching, the Buddha is sharing that one doesn’t get freed from suffering, from discontentment, by having varied experiences or by traveling.

A Bucket of Rice (SN 3.13) - King Pasenadi used a verse given by the Buddha when eating to aid his mindfulness. One can similarly come up with a creative routine as an aid.