Dhammapada · 2.12

Appamādarato bhikkhu, pamāde bhayadassi vā;Abhabbo parihānāya, nibbānasseva santike.

appamadarato bhikkhu, pamade bhayadassi va; abhabbo parihanaya, nibbanasseva santike.

The monk who delights in heedfulness, who sees danger in heedlessness, is incapable of falling away — they are already in the vicinity of Nibbāna.

— Dhammapada 2.12

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.