Acts 1:8 says: But ye shall receive power, after that the Holy Ghost is come upon you: and ye shall be witnesses unto me both in Jerusalem, and in all Judaea, and in Samaria, and unto the uttermost part of the earth.
(KJV).
The verse is spoken by Jesus to his disciples in Acts 1, before his ascension. Read it in context at Acts 1:8, or the full chapter at Acts 1.