Acts 13:2-3—And as they were ministering to the Lord and fasting, the Holy Spirit said, Set apart for Me now Barnabas and Saul for the work to which I have called them. Then, when they had fasted and prayed and laid their hands on them, they sent them away.
For further reading please refer to 倪柝声文集，, vol. 22, “The Prayer Ministry of the Church,” pp. 203-204.