Mark 16:17 Jesus tells us what to look out for in those who believe in Him:
And these signs will accompany those who believe: in my name they will cast out demons; they will speak in new tongues
Acts 2:1-47 The followers of Jesus are filled with the Spirit in the Upper Room at Pentecost:
When the day of Pentecost arrived, they were all together in one place. And suddenly there came from heaven a sound like a mighty rushing wind, and it filled the entire house where they were sitting. And divided tongues as of fire appeared to them and rested on each one of them. And they were all filled with the Holy Spirit and began to speak in other tongues as the Spirit gave them utterance. Now there were dwelling in Jerusalem Jews, devout men from every nation under heaven…[ ]…And at this sound the multitude came together, and they were bewildered, because each one was hearing them speak in his own language…. [ ] …Both Jews and proselytes, Cretans and Arabians—we hear them telling in our own tongues the mighty works of God.
Acts 10:44-46 Gentile believers are filled with the Spirit when Peter visits Caesarea:
While Peter was still saying these things, the Holy Spirit fell on all who heard the word. And the believers from among the circumcised who had come with Peter were amazed, because the gift of the Holy Spirit was poured out even on the Gentiles. For they were hearing them speaking in tongues and extolling God. Then Peter declared…
Acts 11:15 Peter reports back on what happened in Caesarea:
As I began to speak, the Holy Spirit fell on them just as on us at the beginning.
Acts 19:1-7 Paul visits Ephesus:
While Apollos was at Corinth, Paul took the road through the interior and arrived at Ephesus. There he found some disciples and asked them, “Did you receive the Holy Spirit when you believed?” They answered, “No, we have not even heard that there is a Holy Spirit.” So Paul asked, “Then what baptism did you receive?” “John’s baptism,” they replied. Paul said, “John’s baptism was a baptism of repentance. He told the people to believe in the one coming after him, that is, in Jesus.” On hearing this, they were baptized in the name of the Lord Jesus. When Paul placed his hands on them, the Holy Spirit came on them, and they spoke in tongues and prophesied. There were about twelve men in all.
Paul writes most about tongues in Scripture, especially in 1 Corinthians chapters 12-14.
1 Corinthians 12:1
Now concerning spiritual gifts, brothers, I do not want you to be uninformed. You know that when you were pagans you were led astray to mute idols, however you were led. Therefore I want you to understand that no one speaking in the Spirit of God ever says “Jesus is accursed!” and no one can say “Jesus is Lord” except in the Holy Spirit. Now there are varieties of gifts, but the same Spirit; and there are varieties of service, but the same Lord;
1 Corinthians 12:8-11 Paul lists the gifts of the Holy Spirit:
For to one is given through the Spirit the utterance of wisdom, and to another the utterance of knowledge according to the same Spirit, to another faith by the same Spirit, to another gifts of healing by the one Spirit, to another the working of miracles, to another prophecy, to another the ability to distinguish between spirits, to another various kinds of tongues, to another the interpretation of tongues. All these are empowered by one and the same Spirit, who apportions to each one individually as he wills.
1 Corinthians 12:28
And God has appointed in the church first apostles, second prophets, third teachers, then miracles, then gifts of healing, helping, administrating, and various kinds of tongues.
1 Corinthians 13:1
If I speak in the tongues of men and of angels, but have not love, I am a noisy gong or a clanging cymbal.
1 Corinthians 13:8 One day we will have no need for prophecy, tongues or knowledge:
Love never ends. As for prophecies, they will pass away; as for tongues, they will cease; as for knowledge, it will pass away.
1 Corinthians 14:1-10 Paul instructs the church in Corinth:
Pursue love, and earnestly desire the spiritual gifts, especially that you may prophesy. For one who speaks in a tongue speaks not to men but to God; for no one understands him, but he utters mysteries in the Spirit. On the other hand, the one who prophesies speaks to people for their upbuilding and encouragement and consolation. The one who speaks in a tongue builds up himself, but the one who prophesies builds up the church. Now I want you all to speak in tongues, but even more to prophesy. The one who prophesies is greater than the one who speaks in tongues, unless someone interprets, so that the church may be built up. …
1 Corinthians 14:14
For if I pray in a tongue, my spirit prays but my mind is unfruitful. What am I to do? I will pray with my spirit, but I will pray with my mind also; I will sing praise with my spirit, but I will sing with my mind also.
1 Corinthians 14:18
I thank God that I speak in tongues more than all of you.
1 Corinthians 14:39
So, my brothers, earnestly desire to prophesy, and do not forbid speaking in tongues.
Romans 8:26 Paul describes intercession in the Spirit:
Likewise the Spirit helps us in our weakness. For we do not know what to pray for as we ought, but the Spirit himself intercedes for us with groanings too deep for words.
Praying in the Spirit:
But you, beloved, building yourselves up in your most holy faith and praying in the Holy Spirit,
Praying at all times in the Spirit, with all prayer and supplication. To that end keep alert with all perseverance, making supplication for all the saints,
Isaiah prophesies about the gift, hundreds of years before
For by people of strange lips and with a foreign tongue the Lord will speak to this people, to whom he has said, “This is rest; give rest to the weary; and this is repose”; yet they would not hear.
We know that Isaiah is referring to tongues because Paul quotes him in 1 Corinthians 14:
1 Corinthians 14:21-22
In the Law it is written, “By people of strange tongues and by the lips of foreigners will I speak to this people, and even then they will not listen to me, says the Lord.” Thus tongues are a sign not for believers but for unbelievers, while prophecy is a sign not for unbelievers but for believers.