1 Samuel 16:13

Then Samuel took the horn of oil and anointed him in the midst of his brethren, they were witnesses of the solemn act; and the Spirit of the Lord came upon David from that day forward, he received a special endowment with gifts and powers for his royal calling, so that he was gradually led forward to full development, to complete fitness for his life's work. So Samuel rose up and went to Ramah, back to his home. It is probable that the significance of the ceremony was not made known to the other sons of Jesse, who may have been under the impression that David was simply to be regarded as a pupil of the great prophet. But the Lord's will went forward in due time. It happens quite often that the Lord chooses men for His work who are without honor before men. But just such instruments He fills with His Spirit, making them fit to perform the work of their high calling.