Saul lied to Samuel when he said that he had utterly destroyed the Amalekites.
In the book of 1 Samuel in the Old Testament of the Bible, God commanded Saul to completely destroy the Amalekites and all their possessions as punishment for their wickedness. However, Saul and his soldiers spared the king of the Amalekites and some of the best livestock. When Samuel came to confront Saul about this disobedience, Saul claimed that he had obeyed God's command and had completely destroyed the Amalekites.
Samuel, however, knew that Saul was lying and rebuked him for his disobedience. Saul's lie showed that he was more concerned with his own interests than with obeying God's commands. His disobedience and dishonesty ultimately led to his downfall as king.
In summary, Saul lied to Samuel when he claimed to have completely destroyed the Amalekites, even though he had spared the king and some of the best livestock. This lie demonstrated Saul's disobedience and lack of concern for God's commands.