From his Catechetical Lectures, around 347-8:
But there is a second kind of faith, which is bestowed by Christ as a gift of grace. For to one is given through the Spirit the word of wisdom, and to another the word of knowledge according to the same Spirit: to another faith, by the same Spirit, and to another gifts of healing. (1 Corinthians 12:8-9) This faith then which is given of grace from the Spirit is not merely doctrinal, but also works things above man’s power. For whosoever has this faith, shall say to this mountain, Remove hence to yonder place, and it shall remove. (Mark 11:23) For whenever any one shall say this in faith, believing that it comes to pass, and shall not doubt in his heart, then receives he the grace.
And of this faith it is said, If you have faith as a grain of mustard seed. (Matthew 17:20) For just as the grain of mustard seed is small in size, but fiery in its operation, and though sown in a small space has a circle of great branches, and when grown up is able even to shelter the fowls (Matthew 13:32); so, likewise, faith in the swiftest moment works the greatest effects in the soul. For, when enlightened by faith, the soul has visions of God, and as far as is possible beholds God, and ranges round the bounds of the universe, and before the end of this world already beholds the Judgement, and the payment of the promised rewards. Have thou therefore that faith in Him which comes from your own self, that you may also receive from Him that faith which works things above man. (V, 12)