On the last day of the feast, the great day, Jesus stood up and cried out, “If anyone thirsts, let him come to me and drink. 38 Whoever believes in me, as the Scripture has said, ‘Out of his heart will flow rivers of living water.’” Now this he said about the Spirit, whom those who believed in him were to receive, for as yet the Spirit had not been given, because Jesus was not yet glorified. (John 7:37-39)
During his earthly ministry Jesus made a promise that those who believed in Jesus would receive a special blessing, like “rivers of living water”. John interprets this for us and says that Jesus was talking about the Holy Spirit.
In John 20:22 we read that after the resurrection of Jesus (i.e. after Jesus was “glorified”), he breathed on them and the disciples received the Holy Spirit. They were also told to wait in Jerusalem to be clothed with power from on high. This happened in Acts 2.