The baptism of believers in the Holy Spirit is witnessed by the initial physical sign of speaking with other tongues as the Spirit of God gives them utterance.
Acts 2:4 [KJV/NIV]
The speaking in tongues in this instance is the same in essence as the gift of tongues, but is different in purpose and use.
1 Corinthians 12:4-10 [KJV/NIV]
1 Corinthians 12:28 [KJV/NIV]