Session XIII, 15 June 1415, Denzinger no. 626

Whereas in certain parts of the world some rashly presume to assert that the christian people ought to receive the holy sacrament of the Eucharist under both species of bread and wine, and give communion to the lay people everywhere not only under the species of bread but also under the species of wine, and even after supper or otherwise not fasting, and obstinately assert that communion must be given so, against the praiseworthy custom of the Church, reasonably approved, which they try to condemn as sacrilegious: hence it is that this present Council … declares, decrees and defines that, although Christ instituted this venerable sacrament after supper and administered it to his disciples under both species of bread and wine, nevertheless, this notwithstanding, the authority of the sacred canons and the praiseworthy and approved custom of the Church has kept and keeps that this sacrament ought not to be confected after supper, nor received by the faithful when not fasting, except in a case of illness or of other necessity granted or admitted by law or by the Church. And just as this custom was reasonably introduced in order to avoid certain dangers and scandals, so, although in the primitive Church this sacrament was received by the faithful under both species, afterwards it came to be received by those confecting it under both, and by the laity only under the species of bread; since it is to be most firmly believed and in no way doubted that the whole body of Christ and his blood are truly contained under the species of bread as well as under the species of wine. Wherefore to say that to observe this custom or law is sacrilegious or unlawful must be judged erroneous, and those obstinately asserting the opposite of the foregoing are to be restrained as heretics and severely punished by the local diocesan bishops or their officials or by the inquisitors of heretical depravity. …