Albert Barnes Commentary 2 Kings 9:26

"Surely I have seen yesterday the blood of Naboth, and the blood of his sons, saith Jehovah; and I will requite thee in this plat, saith Jehovah. Now therefore take and cast him into the plat [of ground], according to the word of Jehovah." — 2 Kings 9:26 (ASV)

The passage from Surely I have seen to Saith the Lord, is an exegesis of 2 Kings 9:25, containing the “burden” spoken of there.

And the blood of his sons - The murder of Naboth’s sons is mentioned here for the first time; but as the removal of the sons was necessary for the vineyard to pass to Ahab, we can well understand that Jezebel would have taken care to clear them out of the way.