St. Thomas Aquinas
“The greatest devotion is called for at the moment of receiving this sacrament, because it is then that the effect of the sacrament is bestowed, and such devotion is hindered more by what goes before it than by what comes after it. And therefore it was ordained that men should fast before receiving the sacrament rather than after. Nevertheless there ought to be some interval between receiving this sacrament and taking other food. Consequently, both the Postcommunion prayer of thanksgiving is said in the Mass, and the communicants say their own private prayers.”
Source: St. Thomas Aquinas, Summa Theologica