Khuddakavatthukkhandhaka

Vinayapiṭaka · Cūḷavagga (2) · Vol. 7 · Section 147

[147] Now at that time bhikkhus cut their nails with a nail, cut their nails with the mouth, and ground their nails against a wall. Their fingers became painful. They reported this matter to the Blessed One. … "Bhikkhus, I allow a nail-cutter (nakhacchedana)." They cut their nails until they bled; their fingers became painful. … "Bhikkhus, I allow the nails to be cut level with the flesh."

English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.

This passage also appears in 1 other place

Vol. 7 · Section 55 — Khuddakavatthukkhandhaka