John Gill Commentary Matthew 9:29

"Then touched he their eyes, saying, According to your faith be it done unto you." — Matthew 9:29 (ASV)

Then touched he their eyes
Not but that he could have restored sight to them, without touching their eyes, by a word speaking, or by the secret communication of his power; but he might do this as a sign of his favour and kindness to them, and of his will to cure them; as also in compliance with their weakness, who might expect some manual operation upon them;

saying, according to your faith be it unto you :
not that faith in his person and power, was the cause or condition of this cure, or the rule and measure according to which Christ proceeded; but the sense is, that as they had believed he was able to heal them, accordingly a cure should be effected; which, upon his so saying, they immediately found performed in them.