Albert Barnes Commentary 1 Chronicles 28:9

"And thou, Solomon my son, know thou the God of thy father, and serve him with a perfect heart and with a willing mind; for Jehovah searcheth all hearts, and understandeth all the imaginations of the thoughts: if thou seek him, he will be found of thee; but if thou forsake him, he will cast thee off for ever." — 1 Chronicles 28:9 (ASV)

Know thou the God of thy father — "Knowing God," in the sense of having a religious trust in Him, is an unusual phrase in the earlier Scriptures. It scarcely occurs elsewhere in the historical books. David, however, uses the phrase in his Psalms (Psalms 36:10), and its occurrence here may be accepted as evidence that the entire speech is recorded in the actual words of the monarch.