The first example is found in Acts 2, when the one hundred and twenty disciples were baptized in the Holy Spirit on the day of Pentecost:
When the day of Pentecost had come, they were all together in one place. And suddenly there came from heaven a noise like a violent rushing wind, and it filled the whole house where they were sitting. And there appeared to them tongues as of fire distributing themselves, and they rested on each one of them. And they were all filled with the Holy Spirit and began to speak with other tongues, as the Spirit was giving them utterance (Acts 2:1-4; emphasis added).
There is no doubt that the one hundred and twenty believers were already saved and born again before this time, so they definitely experienced the baptism in the Holy Spirit after salvation. It would have been impossible, however, for them to have received the baptism in the Holy Spirit prior to this time simply because the Holy Spirit was not given to the church until that day.
It is obvious that the accompanying sign was speaking with other tongues.