Charles Ellicott Commentary 1 Samuel 17:31

"And when the words were heard which David spake, they rehearsed them before Saul; and he sent for him." — 1 Samuel 17:31 (ASV)

He sent for him. —No doubt much more was said by the brave shepherd boy than the compiler of the history has preserved for us in the brief account here. David felt that supernatural strength had been communicated to him by the Spirit of God, which came upon him on the day of his anointing (1 Samuel 16:13), and it is probable that he had openly avowed his earnest desire of meeting the dreaded foe face to face. This had been reported to Saul.