Moroni, apparently reviewing older records, recorded the words Christ said as he gave the disciples this priesthood power, And he called them by name, saying: Ye shall call on the Father in my name, in mighty prayer; and after ye have done this ye shall have power that to him upon whom ye shall lay your hands, ye shall give the Holy Ghost; and in my name shall ye give it, for thus do mine apostles. Now Christ spake these words unto them at the time of his first appearing; and the multitude heard it not, but the disciples heard it; and on as many as the laid their hands, fell the Holy Ghost (Moroni 2:2-3).
Interestingly, 3 Nephi 18 is likely the text to which Moroni referred before writing Moroni chapters 2-6. These chapters deal with many of the same topics, and reading them together is recommended.