It is for understanding Dukkha that the holy life is lived

Samyutta Nikaya
Translation by Bodhi
81 (8) A Number of Bhikkhus

Then a number of bhikkhus approached the Blessed One … and said to him: [51] “Here, venerable sir, wanderers of other sects ask us: ‘For what purpose, friends, is the holy life lived under the ascetic Gotama?’ When we are asked thus, venerable sir, we answer those wanderers thus: ‘It is, friends, for the full understanding of suffering that the holy life is lived under the Blessed One.’ We hope, venerable sir, that when we answer thus we state what has been said by the Blessed One and do not misrepresent him with what is contrary to fact; that we explain in accordance with the Dhamma, and that no reasonable consequence of our assertion gives ground for criticism.”44
“For sure, bhikkhus, when you answer thus you state what has been said by me and do not misrepresent me with what is contrary to fact; you explain in accordance with the Dhamma, and no reasonable consequence of your assertion gives ground for criticism. For, bhikkhus, it is for the full understanding of suffering that the holy life is lived under me.“ But, bhikkhus, if wanderers of other sects ask you: ‘What, friends, is that suffering for the full understanding of which the holy life is lived under the ascetic Gotama?’—being asked thus, you should answer them thus: ‘The eye, friends, is suffering: it is for the full understanding of this that the holy life is lived under the Blessed One. Forms are suffering … Whatever feeling arises with eye-contact as condition … that too is sufferingThe mind is suffering … Whatever feeling arises with mind-contact as condition … that too is suffering: it is for the full understanding of this that the holy life is lived under the Blessed One. This, friends, is that suffering for the full understanding of which the holy life is lived under the Blessed One.’ [52]
“Being asked thus, bhikkhus, you should answer those wanderers of other sects in such a way.”

Saṁyutta Nikāya 35.81
8. Gilānavagga
Sambahulabhikkhusutta
Variant: Sambahulabhikkhusutta → bhikkhu (pts1ed)
Atha kho sambahulā bhikkhū yena bhagavā tenupasaṅkamiṁsu …pe… ekamantaṁ nisinnā kho te bhikkhū bhagavantaṁ etadavocuṁ:
“idha no, bhante, aññatitthiyā paribbājakā amhe evaṁ pucchanti:
‘kimatthiyaṁ, āvuso, samaṇe gotame brahmacariyaṁ vussatī’ti?
Evaṁ puṭṭhā mayaṁ, bhante, tesaṁ aññatitthiyānaṁ paribbājakānaṁ evaṁ byākaroma:
dukkhassa kho, āvuso, pariññatthaṁ bhagavati brahmacariyaṁ vussatī’ti.

Kacci mayaṁ, bhante, evaṁ puṭṭhā evaṁ byākaramānā vuttavādino ceva bhagavato homa, na ca bhagavantaṁ abhūtena abbhācikkhāma, dhammassa cānudhammaṁ byākaroma, na ca koci sahadhammiko vādānuvādo gārayhaṁ ṭhānaṁ āgacchatī”ti?

“Taggha tumhe, bhikkhave, evaṁ puṭṭhā evaṁ byākaramānā vuttavādino ceva me hotha, na ca maṁ abhūtena abbhācikkhatha, dhammassa cānudhammaṁ byākarotha, na ca koci sahadhammiko vādānuvādo gārayhaṁ ṭhānaṁ āgacchati.
Dukkhassa hi, bhikkhave, pariññatthaṁ mayi brahmacariyaṁ vussati.

Sace pana vo, bhikkhave, aññatitthiyā paribbājakā evaṁ puccheyyuṁ:
‘katamaṁ pana taṁ, āvuso, dukkhaṁ, yassa pariññāya samaṇe gotame brahmacariyaṁ vussatī’ti?
Evaṁ puṭṭhā tumhe, bhikkhave, tesaṁ aññatitthiyānaṁ paribbājakānaṁ evaṁ byākareyyātha:
‘cakkhu kho, āvuso, dukkhaṁ, tassa pariññāya bhagavati brahmacariyaṁ vussati.
Rūpā …pe…
yampidaṁ cakkhusamphassapaccayā uppajjati vedayitaṁ sukhaṁ vā dukkhaṁ vā adukkhamasukhaṁ vā tampi dukkhaṁ.
Tassa pariññāya bhagavati brahmacariyaṁ vussati …pe…

mano dukkho …pe…
yampidaṁ manosamphassapaccayā uppajjati vedayitaṁ sukhaṁ vā dukkhaṁ vā adukkhamasukhaṁ vā tampi dukkhaṁ.
Tassa pariññāya bhagavati brahmacariyaṁ vussati.

Idaṁ kho taṁ, āvuso, dukkhaṁ, tassa pariññāya bhagavati brahmacariyaṁ vussatī’ti.
Evaṁ puṭṭhā tumhe, bhikkhave, tesaṁ aññatitthiyānaṁ paribbājakānaṁ evaṁ byākareyyāthā”ti.