I confess I have never heard this piece before . . .
It is called Miserere Mei Deus ("Have mercy on me, O God"), a setting of Psalm 51, by Gregorio Allegri. "The Miserere is written for two choirs, one of five and one of four voices, and is generally accepted to be one of the finest examples of Renaissance polyphony to survive to the present day." (Wikipedia)
Text and Translation
(HT: Whispers . . .)