"You perhaps say: 'My bread is usual.' But the bread is bread before the words of the sacraments; when consecration has been added, from bread it becomes the flesh of Christ.
So let us confirm this, how it is possible that what is bread is the body of Christ. By what words, then, is the consecration and by whose expressions? By those of the Lord Jesus.
For all the rest that are said in the preceding are said by the priest: praise to God, prayer is offered, there is a petition for the people, for kings, for the rest. When it comes to performing a venerable sacrament, then the priest uses not his own expressions, but he uses the expressions of Christ.
Thus the expression of Christ performs this sacrament."
Ambrose - "The Sacraments" Book 4, Ch.4:14.