But he was a prophet and knew that God had promised him on oath that He would place one of his descendants on his throne.
KJV
Therefore being a prophet, and knowing that God had sworn with an oath to him, that of the fruit of his loins, according to the flesh, he would raise up Christ to sit on his throne;
ASV
Being therefore a prophet, and knowing that God had sworn with an oath to him, that of the fruit of his loins he would set one upon his throne;