Council of Trent: Session XXII, Sacrifice of the Mass, Chapter VII

CHAPTER VII. On the water that is to be mixed with the wine to be offered in the chalice.

The holy Synod notices, in the next place, that it has been enjoined by the Church on priests, to mix water with the wine that is to be offered in the chalice; as well because it is believed that Christ the Lord did this, as also because from His side there came out blood and water; the memory of which mystery is renewed by this commixture; and, whereas in the apocalypse of blessed John, the peoples are called waters, the union of that faithful people with Christ their head is hereby represented.

Translation: J. Waterworth, The Canons and Decrees of the Council of Trent (1848), Hanover Historical Texts Project transcription (1995).