16 “And My prayer shall return into Mine Own Bosom.” In the bosom of this verse is plainly a great depth, and may the Lord grant that it be fathomable by us. For in the “bosom” a secret is understood. And we ourselves, Brethren, are here well admonished to pray within our own bosom, where God sees, where God hears, where no human eye penetrates, where none sees but He who succours; where Susanna prayed, and her voice, though it was not heard by men, yet by God was heard....We read also that in the mount Jesus prayed alone, we read that He passed the night in prayer, even at the time of His Passion. What then?
“And My prayer shall return into Mine Own Bosom.” I know not what better to understand concerning the Lord: take meanwhile what now occurs; perhaps something better will occur hereafter, either to me or to some better: “My prayer shall return into Mine Own Bosom:” this I understand to be said, because in His Own Bosom He had the Father. “For God was in Christ reconciling the world unto Himself.” In Himself He had Him to whom He prayed. He was not far from Him, for Himself had said, “I am in the Father, and the Father in Me.” But because prayer rather belongs to very Man (for according as Christ is the Word, He prays not, but hears prayer; and seeks not to be succoured for Himself, but with the Father succours all): what is, “My prayer shall return into Mine Own Bosom,” but in Me My Manhood invokes in Me My Godhead.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)