Hebrews 1:5

Unto which of the angels] Angels are sometimes in the OT. called 'sons of Elohim,' e.g. in Job 1:6, i.e. belonging to the class 'Elohim: cp. 'sons of the prophets,' i.e. members of the prophetical class. 'Elohim,' at first a plural, meaning spiritual beings, was used as a singular to signify the one God, As used of the angels, the term does not imply sonship in the unique sense in which it is used of Christ. The first citation is from Psalms 2:7, commonly interpreted as Messianic, and accepted as such by the writer and his readers. The second is from 2 Samuel 7:14 and Psalms 89:26. God's promise to the line of Davidic kings is here applied to Messiah, as Son of David.