Khuddakavatthukkhandhaka

Vinayapiṭaka · Cūḷavagga (2) · Vol. 7 · Section 16

[16] Now at that time a certain bhikkhu had a sore on his face. He asked the bhikkhus, "Friends, what is my sore like?" The bhikkhus said, "Friend, your sore is like this." He did not believe them. They reported this matter to the Blessed One. … "Bhikkhus, I allow, on account of illness, looking at the reflection of one's face in a mirror or in a bowl of water."

English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.