Albert Barnes Commentary Psalms 40:8

"I delight to do thy will, O my God; Yea, thy law is within my heart." — Psalms 40:8 (ASV)

I delight to do thy will, O my God – That is, in obeying the law, in submitting to all the trials appointed to me, and in making an atonement for the sins of men. See the notes at Hebrews 10:7. Compare Philippians 2:8; Matthew 26:39.

Yea, thy law is within my heart – Margin, “In the midst of my bowels.” So the Hebrew. The idea is, that the law of God was within him. His obedience was not external but proceeded from the heart. How true this was of the Redeemer it is not necessary here to say.