1 Samuel 19:21
And when it was told Saul, he sent other messengers, and they prophesied likewise. And Saul sent messengers again the third time, and they prophesied also. And when it was told Saul, he sent other messengers, and they prophesied likewise. And Saul sent messengers again the third time, and they prophesied also.
Saul was pursuing David to seize him and kill him, sending successive groups of messengers after the first group to Ramah, where Samuel lived and the company or school of the prophets was. When the second and third groups reached the prophets and came into contact with and under the influence of the school, they were overcome by the Spirit of God and, like those sent before them, joined the first group and the prophets in prophesying, and this happened repeatedly. Their prophetic words proclaimed what would happen in the future and the glory and majesty of Jehovah.
The school where this happened trained people in study of Scripture and praise to God, and its members were by themselves in prayer until the Holy Spirit rested upon them. That preparation through the Spirit's endowment fitted them to go out and carry their testimony and the light God had given them to people in spiritually dark places and to those willing to listen. Saul himself also came to the school, the Holy Spirit came upon him, and he was moved to prophesy with the prophets, temporarily forgetting his hostility toward David and being diverted from pursuing him, so the pursuit did not succeed.
Saul did not hold on to that changed spirit or remain under the Spirit's influence, but later returned to trying to kill David. A moment of the Spirit's influence is not a substitute for enduring character. The experience shows God restraining Saul's evil purpose and protecting David.