Dhamma Verses (Dhammapada, KN 2)

3: The Chapter about the Mind – 43
The mind should be well guarded
Anupubba was a faithful householder who fulfilled his duties
to the monastics, and later was ordained himself, but became
weary with all the rules and regulations; the Buddha told him
his real duty was only to guard his mind, and further taught
him with this verse.
36. Sududdasaṁ sunipuṇaṁ yatthakāmanipātinaṁ,
cittaṁ rakkhetha medhāvī, cittaṁ guttaṁ sukhāvahaṁ.
Hard to see and very subtle,
and flitting wherever it will,
the sage should surely guard the mind,
a guarded mind brings happiness.