As with the Buddha’s last words before his parinibbāna:
Then the Blessed One addressed the bhikkhus thus: “Indeed, bhikkhus, I declare this to you: It is in the nature of all formations to dissolve. Attain perfection through diligence.” These were the Blessed One’s last words.
Atha kho bhagavā bhikkhū āmantesi: “handa dāni, bhikkhave, āmantayāmi vo, vayadhammā saṅkhārā appamādena sampādethā”ti. Ayaṃ tathāgatassa pacchimā vācā.
— D.16, Mahāparinibbāna Sutta
And the highest importance of heedfulness (appamāda):
“Bhikkhus, to whatever extent there are beings, whether footless or with two feet, four feet, or many feet, whether having form or formless, whether percipient or non-percipient, or neither percipient nor non-percipient, the Tathāgata, the Arahant, the Perfectly Enlightened One is declared foremost among them. So too, all wholesome qualities are rooted in heedfulness and converge upon heedfulness and heedfulness is declared foremost among them.”
Yāvatā, bhikkhave, sattā apadā vā dvipadā vā catuppadā vā bahuppadā vā rūpino vā arūpino vā saññino vā asaññino vā nevasaññināsaññino vā, tathāgato tesaṃ aggamakkhāyati arahaṃ sammāsambuddho; evamevaṃ kho, bhikkhave, ye keci kusalā dhammā, sabbe te appamādamūlakā appamādasamosaraṇā. Appamādo tesaṃ aggamakkhāyati.
— AN 10.15, Appamāda Sutta