17 The following verses, which are sung in praise of Him when Allelujah is chanted, show how He used this hatred of theirs, both for the trial of His own people, and for the glory of His Name, which is profitable for us. “He sent Moses His servant, and Aaron whom He had chosen him”. “Whom He had chosen,” would be sufficient; but there is no difficulty in the addition of “him.” It is a phrase of Scripture, as, “The land in which they shall dwell in it:” a phrase which the divine pages are full of.
18. “He set forth in them the words of His tokens, and of His wonders in the land of Ham”. We ought not to understand by “the words of His tokens,” words literally, words with which the tokens and wonders were worked, that is, which they uttered, that these tokens and wonders might take place. For many were performed without words, either with a rod, or with outstretched hand, or by ashes sent towards heaven....
19. “He sent darkness, and made it dark”. This is also written among the plagues with which the Egyptians were smitten. But what follows, is variously read in different copies. For some have, “and they provoked His words;” while others read, “and they provoked not His words;” but the reading first mentioned we have found in most; while, where the negative particle is added, we could hardly discover two copies. But perhaps the false reading has abounded owing to the easy sense; for what is easier understood than this, “They provoked His words,” that is, by their contumacious rebellions? We have endeavoured to explain the other reading also according to some true sense: and this for the present occurs: “They provoked not His words,” that is, in Moses and Aaron; because they most patiently bore with a very stiffnecked people, until all things which God had determined to work by them, were fulfilled in order.
20. “He turned their waters into blood, and slew their fish”. “He made their land frogs, yea, even in the king's chambers”: as if he were to say, He turned their land into frogs. For there was so great a multitude of frogs, that this might well be said by hyperbole.
21. “He spoke the word, and there came all manner of flies, and lice in all their quarters”. If it be asked when He spoke, it was in His Word before it took place; and there it was, without time, at what time it should take place: although even then He commanded it to be done, when it was to be done, through Angels, and through his servants Moses and Aaron.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)