Charles Ellicott Commentary Psalms 103:20

"Bless Jehovah, ye his angels, That are mighty in strength, that fulfil his word, Hearkening unto the voice of his word." — Psalms 103:20 (ASV)

Just as in the highest revelation made by Jesus Christ the angels in heaven rejoice over the repentant sinner, so in the psalmist’s view the mercy of Jehovah to his faithful people is cause for high acclaim among the hosts around the throne.