Council of Trent: Session XIII, Eucharist, Chapter VII

CHAPTER VII. On the preparation to be given that one may worthily receive the sacred Eucharist.

If it is unbeseeming for any one to approach to any of the sacred functions, unless he approach holily; assuredly, the more the holiness and divinity of this heavenly sacrament are understood by a Christian, the more diligently ought he to give heed that he approach not to receive it but with great reverence and holiness, especially as we read in the Apostle those words full of terror; He that eateth and drinketh unworthily, eateth and drinketh judgment to himself. Wherefore, he who would communicate, ought to recall to mind the precept of the Apostle; Let a man prove himself. Now ecclesiastical usage declares that necessary proof to be, that no one, conscious to himself of mortal sin, how contrite soever he may seem to himself, ought to approach to the sacred Eucharist without previous sacramental confession. This the holy Synod hath decreed is to be invariably observed by all Christians, even by those priests on whom it may be incumbent by their office to celebrate, provided the opportunity of a confessor do not fail them; but if, in an urgent necessity, a priest should celebrate without previous confession, let him confess as soon as possible.

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