John Gill Commentary 1 Kings 1:30

"verily as I sware unto thee by Jehovah, the God of Israel, saying, Assuredly Solomon thy son shall reign after me, and he shall sit upon my throne in my stead; verily so will I do this day." — 1 Kings 1:30 (ASV)

Even as I swore to you by the Lord God of Israel
And so owns and confirms the truth of what Nathan had suggested to Bathsheba, and she had asserted, (1 Kings 1:13 1 Kings 1:17) ;

saying, assuredly Solomon your son shall reign after me, and he shall
sit upon my throne in my stead ;
this was the substance of the oath:

even so will I certainly do this day ;
perform this oath, and set Solomon on the throne.