"I want all of you to speak in tongues." (Paul, in 1Cor.14:5)
If we believe Paul's words, then we must admit that there is such an experience as being empowered by God's holy Spirit to speak in a language which we have not learned. The Scriptures bear indisputable witness to the presence of this experience in the early church. Beginning in the second chapter of Acts and throughout the New Testament writings, there are references to this extraordinary blessing among both Jewish and Gentile believers.

A few years after the initial outpouring of the Spirit on the Jews at Pentecost, God began admitting Gentiles into the church by baptizing them with the holy Ghost. This grace, which Jewish believers did not expect to be shown to Gentiles, is recorded in Acts 10:45-46: "All the believing Jews who came with Peter were astonished because the gift of the holy Ghost was poured out on the Gentiles also; for they heard them speaking in tongues and praising God." Later, another group of Gentiles spoke in tongues when they received the Spirit: "And when Paul had laid his hands upon them, the holy Ghost came on them, and they spoke in tongues and prophesied" (Acts 19:6). These were not isolated events; on the contrary, the context in Acts 19 suggests that Paul was surprised that these believers had not already received the baptism of the Spirit. Further, in all his letters which speak of the subject, Paul speaks of this blessing as if it were a normal part of church life.

In reality, there is no church life without the language of the Spirit, for receiving the holy Spirit and speaking in tongues are indissolubly united as two parts of one experience in Christ. The moment one receives the holy Ghost, he is moved by the Spirit to speak in a language he has not learned. This is true in every case, as Jesus explained to Nicodemus (Jn.3:8). When all the evidence is rightly divided and justly weighed, it becomes clear that every person who receives the holy Ghost speaks in tongues when he receives it, and that if one has not spoken in tongues, then he has no basis on which to claim that he has received the Spirit of God.

The only way to receive the holy Ghost, my friend, is by repentance and faith toward God; and the divinely ordained proof that this has happened is tongues. Jesus said it this way, "When the Comforter is come, whom I will send unto you from the Father, even the Spirit of truth, which proceeds from the Father, he shall testify of me" (Jn.15:26). Or as Paul would later phrase it, "The Spirit itself bears witness with our spirit that we are children of God

This is an in-depth teaching on the gift of tongues.

http://www.layhands.com/prayinginthespirit.htm