1 Samuel 15:10-11
10-11Then God spoke to Samuel: "I'm sorry I ever made Saul king. He's turned his back on me. He refuses to do what I tell him."
11-12Samuel was angry when he heard this. He prayed his anger and disappointment all through the night. He got up early in the morning to confront Saul but was told, "Saul's gone. He went to Carmel to set up a victory monument in his own honor, and then was headed for Gilgal."
By the time Samuel caught up with him, Saul had just finished an act of worship, having used Amalekite plunder for the burnt offerings sacrificed to God.