7 And not in vain is my warning turned to this point; for I hear that Sarmatio and Barbatianus have come to you, foolish talkers, who say that there is no merit in abstinence, no grace in a frugal life, none in virginity, that all are valued at one price, that they are mad who chasten their flesh with fastings, that they may bring it into subjection to the spirit. But if he had thought it madness, Paul the Apostle would never himself have acted thus, nor written to instruct others. For he glories in it, saying: “But I chasten my body, and bring it into bondage, lest, after preaching to others, I myself should be found reprobate.” So they who do not chasten their body, and desire to preach to others, are themselves esteemed reprobates.
Source: Letters (New Advent)