Dhammapada · 3.10

Diso disaṃ yaṃ taṃ kayirā, verī vā pana verinaṃ;Micchāpaṇihitaṃ cittaṃ, pāpiyo naṃ tato kare.

diso disam yam tam kayira, veri va pana verinam; miccha-panihitam cittam, papiyo nam tato kare.

Whatever harm an enemy might do to an enemy, or one hater to another — a wrongly-directed mind would do far worse to its owner.

— Dhammapada 3.10

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.