Jamieson, Fausset and Brown on 1 Samuel 10:9-11

9-11. when he had turned his back to go from Samuel, God gave him another heart--Influenced by the words of Samuel, as well as by the accomplishment of these signs, Saul's reluctance to undertake the onerous office was overcome. The fulfilment of the two first signs [1Sa 10:7, 8] is passed over, but the third is specially described. The spectacle of a man, though more fit to look after his father's cattle than to take part in the sacred exercises of the young prophets--a man without any previous instruction, or any known taste, entering with ardor into the spirit, and skilfully accompanying the melodies of the sacred band, was so extraordinary a phenomenon, that it gave rise to the proverb, "Is Saul also among the prophets?" (see 1Sa 19:24). The prophetic spirit had come upon him; and to Saul it was as personal and experimental an evidence of the truth of God's word that had been spoken to him, as converts to Christianity have in themselves from the sanctifying power of the Gospel.