Samuel told Saul the LORD had sought "a man after His own…" what?

Answer: Heart

Because Saul had not kept the LORD's command, his kingdom would not endure; the LORD had sought a man after His own heart.

“But now your kingdom will not endure; the LORD has sought a man after His own heart and appointed him ruler over His people, because you have not kept the command of the LORD.””— 1 Samuel 13:14 (BSB)