21 Say then, Lord our God, what do You enjoin your people, Your Israel? “Immolate to God the sacrifice of praise”. Let us also say to Him, “In me, O God, are your vows, which I will render of prose to You.” I had feared lest You might enjoin something which would be out of my power, which I was counting to be in my pen, and but now perchance it had been taken away by a thief. What do You enjoin me? “Immolate to God the sacrifice of praise.” Let me revert to myself, wherein I may find what I may immolate: let me revert to myself; in myself may I find immolation of praise: be Your altar my conscience.
We are without anxiety, we go not into Arabia in quest of frankincense: not any bags of covetous dealer do we sift: God requires of us the sacrifice of praise. Zacchæus had the sacrifice of praise in his patrimony; the widow had it in her bag; some poor host or other has had it in his jar: another neither in patrimony, nor in bag, nor in jar, has had anything, had it wholly in his heart: salvation was to the house of Zacchæus; and more this poor widow cast in than those rich men: this man, that does offer a cup of cold water, shall not lose his reward: but there is even “peace on earth to men of good will.” “Immolate to God the sacrifice of praise.”
O sacrifice gratuitous, by grace given! I have not indeed bought this to offer, but You have given: for not even this should I have had. And this is the immolation of the sacrifice of praise, to render thanks to Him from whom you have whatever of good you have, and by whose mercy is forgiven you whatsoever of evil of yours you have. “Immolate to God the sacrifice of praise: and render to the Highest your prayers.” With this odour the Lord is well pleased.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)