A.T. Robertson Commentary Hebrews 1:5

"For unto which of the angels said he at any time, Thou art my Son, This day have I begotten thee? and again, I will be to him a Father, And he shall be to me a Son?" — Hebrews 1:5 (ASV)

Unto which (Τιν). "To which individual angel." As a class angels are called sons of God (Elohim) (Psalms 29:1), but no single angel is called God's Son like the Messiah in Ps 2:7. Dods takes "have I begotten thee" (γεγεννηκα σε, perfect active indicative of γενναω) to refer to the resurrection and ascension while others refer it to the incarnation.

And again (κα παλιν). This quotation is from 2 Samuel 7:14. Note the use of εις in the predicate with the sense of "as" like the Hebrew (LXX idiom), not preserved in the English. See Mt 19:5; Luke 2:34. Like Old English "to" or "for." See 2 Corinthians 6:18; Revelation 21:7 for the same passage applied to relation between God and Christians while here it is treated as Messianic.