A.T. Robertson Commentary Acts 2:34

"For David ascended not into the heavens: but he saith himself, The Lord said unto my Lord, Sit thou on my right hand," — Acts 2:34 (ASV)

Ascended not (ου--ανεβη). It is more emphatic than that: For not David ascended into the heavens. Peter quotes Ps 110:1 as proof. No passage in the O.T. is so constantly quoted as Messianic as this. "St. Peter does not demand belief upon his own assertion, but he again appeals to the Scriptures, and to words which could not have received a fulfilment in the case of David" (Knowling).

Sit thou (καθου). Late Koine form for earlier καθησο, present middle imperative second singular of καθημα.