Albert Barnes Commentary 1 Chronicles 29:20

"And David said to all the assembly, Now bless Jehovah your God. And all the assembly blessed Jehovah, the God of their fathers, and bowed down their heads, and worshipped Jehovah, and the king." — 1 Chronicles 29:20 (ASV)

Worshipped the Lord, and the king - The same outward signs of reverence were accorded by the customs of the Jews (as with other Eastern nations) to both God and their monarchs (see 1 Kings 1:31). But the application of these terms to both in the same passage, which occurs nowhere else in Scripture, is thought to indicate a time when long servitude under despotic rulers had influenced their manner of speech to reflect Eastern customs.