Bede's Ecclesiastical History of England, ed. by A.M. Sellar, , at sacred-texts.com
At which time he also sent Augustine a letter concerning the miracles that he had heard had been wrought by him; wherein he admonishes him not to incur the danger of being puffed up by the number of them. The letter was in these words:
"I know, dearly beloved brother, that Almighty God, by means of you, shows forth great miracles to the nation which it was His will to choose. Wherefore you must needs rejoice with fear, and fear with joy concerning that heavenly gift; for you will rejoice because the souls of the English are by outward miracles drawn to inward grace; but you will fear, lest, amidst the wonders that are wrought, the weak mind may be puffed up with self-esteem, and that whereby it is outwardly raised to honour cause it inwardly to fall through vain-glory. For we must call to mind, that when the disciples returned with joy from preaching, and said to their Heavenly Master, ‘Lord, even the devils are subject to us through Thy Name;’ forthwith they received the reply, ‘In this rejoice not; but rather rejoice, because your names are written in heaven.’ For their minds were set on private and temporal joys, when they rejoiced in miracles; but they are recalled from the private to the common joy, and from the temporal to the eternal, when it is said to them, ‘Rejoice in this, because your names are written in heaven.’ For all the elect do not work miracles, and yet the names of all are written in heaven. For those who are disciples of the truth ought not to rejoice, save for that good thing which all men enjoy as well as they, and in which their joy shall be without end.
"It remains, therefore, most dear brother, that amidst those outward actions, which you perform through the power of the Lord, you should always carefully judge yourself in your heart, and carefully understand both what you are yourself, and how much grace is bestowed upon that same nation, for the conversion of which you have received even the gift of working miracles. And if you remember that you have at any time sinned against our Creator, either by word or deed, always call it to mind, to the end that the remembrance of your guilt may crush the vanity which rises in your heart. And whatsoever gift of working miracles you either shall receive, or have received, consider the same, not as conferred on you, but on those for whose salvation it has been given you."