In the Gospel of John the Baptist sees Jesus approaching and says, behold the Lamb of God, who takes away the sin of the world. The Roman liturgy took his words and set them at the breaking of the bread. The Liber Pontificalis, the old book of papal lives, records that Pope Sergius I, elected in 687 and himself of Syrian descent, decreed that at the time of the fraction the clergy and people should sing Agnus Dei, qui tollis peccata mundi, miserere nobis, and scholars have long connected the innovation with the eastern Christian communities then prominent in Rome, and with Sergius's known resistance to a council that had restricted depicting Christ as a lamb.
The chant accompanied the practical business of breaking the consecrated bread for distribution, and was repeated as long as the fraction took, before settling into the familiar threefold form, twice have mercy on us, and the third time grant us peace, a petition matching the kiss of peace nearby in the rite. In Masses for the dead the older books changed the endings to grant them rest, dona eis requiem, with eternal rest at the third repetition, a variant every listener to the great requiem settings has heard even if they never met it in a liturgy.