He [Jesus] said to them: "It is not for you to know the times or dates the Father has set by His own authority. But you will receive power when the Holy Spirit comes upon you, and you will be my witnesses in Jerusalem, and in all Judea and Samaria, and to the ends of the earth."
Suddenly a sound like the blowing of a violent wind came from heaven and filled the whole house where they were sitting. They saw what seemed to be tongues of fire that separated and came to rest on each of them. All of them were filled with the Holy Spirit and began to speak in other tongues as the Spirit enabled them.
Acts 1:7-8 (NIV)
Acts 2:2-4 (NIV)