Reference for Psalms 103:20

בָּרֲכ֥וּ יְהוָ֗ה מַלְאָ֫כָ֥יו גִּבֹּ֣רֵי כֹ֭חַ עֹשֵׂ֣י דְבָר֑וֹ לִ֝שְׁמֹ֗עַ בְּק֣וֹל דְּבָרֽוֹ׃

Bless the LORD, ye angels of His, Ye mighty in strength, that fulfil His word, Hearkening unto the voice of His word.

ספר השרשים