Dhammapada · 25.21

Attā hi attano nātho, attā hi attano gati;Tasmā saṃyamamattānaṃ, assaṃ bhadraṃva vāṇijo.

Atta hi attano natho, atta hi attano gati; Tasma samyamamattanam, assam bhadramva vanijo.

The self is the lord of the self; the self is the refuge of the self. Therefore restrain yourself as a merchant restrains a fine horse.

— Dhammapada 25.21

Source: Pali text from the Khuddaka Nikāya of the Pali Canon (oldest extant Buddhist scripture, compiled c. 3rd century BCE). Public-domain English readings drawn from F. Max Müller's translation in Sacred Books of the East, Vol X (1881) and the Buddhist Publication Society's freely-licensed Acharya Buddharakkhita rendering.