14 “In the head of the Book it is written of me, that I should fulfil Your will: O my God, I am willing, and Your Law is within my heart”. Behold! He turns His regards to His members. Behold! He has Himself “fulfilled the will” of the Father. But in what “beginning of a Book” is it written of Him? Perhaps in the beginning of this Book of Psalms. For why should we seek far for it, or examine into other books for it? Behold! It is written in the beginning of this Book of Psalms! “His will is in the Law of the Lord;” that is, “'O my God, I am willing,' and 'Your Law is within my heart;'” that is the same as, “And in His Law does he meditate day and night.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)