On the Institution of the Eucharist (my emphases):
“I speak not only to the communicant, but also I say to the priest who ministers the Sacrament: Distribute this gift with much care. There is no small punishment for you, if being conscious of any wickedness in any man, you allow him to partake of the banquet of the table: ‘Shall I not now require his blood at your hand?’ (2 Sam. 4:11). If some public figure, or some wealthy person who is unworthy, presents himself to receive Holy Communion, forbid him. The receive? I am speaking here not of some unknown sinner, but of a notorious one. If someone who is not a disciple, through ignorance, comes to Communion, do not be afraid to forbid him. Fear God, not man. If you fear man, you will be scorned and laughed at even by him; but if you fear God, you will be an object of respect even to men. But if you cannot do it, bring that sinner to me, for I will not allow anyone to dare do these things. I would give up my life rather than give the Lord’s Blood to the unworthy.
“If, however, a sinful person receives Communion, and you did not know his character, you are not to blame, however. I say the things above concerning only those who sin openly. For if we amend these, God will speedily reveal to us the unknown also; but if we let these flagrant abuses continue, how can we expect Him to make manifest those that are hidden? I say these things, not to repel sinners or cut them off, but I say it in order that we may bring them to repentance, and bring them back, so that we may take care of them. For thus we shall both please God and lead many to receive worthily. And for our own diligence, and for our care for others, we will receive a great reward. May we attain that reward by the grace and love that God gives to man through Our Lord Jesus Christ, to whom be glory, world without end. Amen.”