A.T. Robertson Commentary Hebrews 1:13

"But of which of the angels hath he said at any time, Sit thou on my right hand, Till I make thine enemies the footstool of thy feet?" — Hebrews 1:13 (ASV)

Hath he said (ειρηκεν). Perfect active common use of the perfect for permanent record. This seventh quotation is proof of the Son's superiority as the Son of God (his deity) to angels and is from Ps 110:1, a Messianic Psalm frequently quoted in Hebrews.

Sit thou (καθου). Second person singular imperative middle of καθημα, to sit, for the longer form καθησο, as in Mt 22:44; James 2:3.

On my right hand (εκ δεξιων μου). "From my right." See 1:3 for εν δεξια "at the right hand."

Till I make (εως αν θω). Indefinite temporal clause about the future with εως and the second aorist active subjunctive of τιθημ with αν (often not used), a regular and common idiom. Quoted also in Lu 20:43. For the pleonasm in υποδιον and των ποδων (objective genitive) see Mt 5:35.