Aññatitthiyavagga

Suttapiṭaka · Saṁyuttanikāya · Mahāvaggasaṁyutta · Vol. 19 · Section 117

[117] Setting at Sāvatthī. Bhikkhus, if wanderers of other sects (aññatitthiyā paribbājakā) should ask you thus: "For what purpose, friends, is the holy life (brahmacariya) lived under the ascetic Gotama?" — being asked thus, bhikkhus, you should answer those wanderers of other sects thus: "It is for the fading away of lust (rāgavirāga), friends, that the holy life is lived under the Blessed One."

English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.

This passage also appears in 1 other place

Vol. 19 · Section 25 — Kimatthiyasutta