John Chrysostom
Catena Aurea · Matthäusevangelium 12
Either make the tree good, and his fruit good; or else make the tree corrupt, and his fruit corrupt: for the tree is known by his fruit. O generation of vipers, how can ye, being evil, speak good things? for out of the abundance of the heart the mouth speaketh. A good man out of the good treasure of the heart bringeth forth good things: and an evil man out of the evil treasure bringeth forth evil things.
Herein also He shews His Godhead as knowing the hidden things of the heart; for not for words only, yea but for evil thoughts also they shall receive punishment. For it is the order of nature that the store of the wickedness which abounds within should be poured forth in words through the mouth. Thus when you shall hear any speaking evil, you must infer that his wickedness is more than what his words express; for what is uttered without is but the overflowing of that within; which was a sharp rebuke to them. For if that which was spoken by them were so evil, consider how evil must be the root from whence it sprung. And this happens naturally; for oftentimes the hesitating tongue does not suddenly pour forth all its evil, while the heart, to which none other is privy, begets whatsoever evil it will, without fear; for it has little fear of God. But when the multitude of the evils which are within is increased, the things which had been hidden then burst forth through the mouth. This is that He says, Out of the abundance of the heart the mouth speaketh.