Psalms 103:20

Psalms 103:20. Bless the LORD, ye his angels, that excel in strength, that do his commandments, hearkening unto the voice of his word.

David calls in the angels to help him to praise the Lord; he wants to do it well, but feels that he is weak and feeble, so he calls in the best of help.

We also sing, -

"Angels, assist our mighty joys,

Strike all your harps of gold;

But when you raise your highest notes,

His love can ne’er be told."