3 Think thou, soul, of all the rewards of God, in thinking over all your wicked deeds: for as many as are your sins, so many are His rewards of good. And what present, what offering, what sacrifice, can you ever tender unto Him?...What will you reward the Lord with? For you were reflecting, and couldest not find: “I will receive the cup of salvation.” What? Has not the Lord Himself given the cup of salvation? Reward Him from your own, if you can? I would say, No, do it not; reward Him not from your own; God does not will to be rewarded from your own.
If you reward Him from your own, you reward sin. For all that you have you have from Him: sins only you have of your own. He does not wish to be rewarded from yours, He does will from His own. Just as, if you should bring to a husbandman, from the land which he has sown, an ear of wheat, you have rewarded him from the husbandman's own produce; if thorns, that hast offered him of your own. Reward truth, in truth praise the Lord: if you shall choose to reward Him from your own, you will lie.
He who speaks a lie, speaks of his own. If he who speaks a lie, speaks of his own: so he who speaks truth, speaks of the Lord's. But what is to receive the cup of salvation, but to imitate the Passion of our Lord?...I will receive the cup of Christ, I will drink of our Lord's Passion. Beware that thou fail not. But, “I will call upon the Name of the Lord.” They then who failed, called not upon the Lord; they presumed in their own strength. Do thou so return, as remembering that you are returning what you have received. So then let your soul bless the Lord, as not to forget all His rewards.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)