Bede the Venerable — as excerpted in the Catena Aurea on Mark 9:14-29

“(ubi sup.) The answer of the Lord was suited to the petition; for the man said, If thou canst do any thing, help us; and to this the Lord answered, If thou canst believe. On the other hand, the leper who cried out, with faith, Lord, if thou will, thou canst make me clean, (Matt. 8:2. 3) received an answer according to his faith, I will, be thou clean.”