Singing Worthily

John Calvin wrote in his 1542 epistle to the reader of the Genevan psalter that:

Now what Saint Augustine says is true, that no one is able to sing things worthy of God unless he has received them from him. Wherefore, when we have looked thoroughly everywhere and searched high and low, we shall find no better songs nor more appropriate for the purpose than the Psalms of David, which the Holy Spirit made and spoke through him. And furthermore, when we sing them, we are certain that God puts the words in our mouths, as if he himself were singing in us to exalt his glory.

Now it may be inquired, Where did Augustine say that? Hughes Oliphant Old, The Patristic Roots of Reformed Worship, pp. 262-263, points us to Augustine's commentary on Psalm 35, where we find these remarks on the first verse:

The psalm belongs to Him: our heart is His, let our tongue sing appropriate things to Him: if only he shall deign to give that which the tongue should sing. No one sings to Him except he who would receive from Him what he should be able to sing. Finally we are able to sing in this way because He has spoken by his Spirit through the Prophet and in these words we recognize Him also.