Alma 18

As Ammon is out tending the king's flock with the king's servants, having had the flock scattered my men at the waters of Sebus, Ammon sees a great opportunity to gain the allegiance of the servants, and ponders -

"...I will show forth my power unto these my fellow–servants, or the power which is in me, in restoring these flocks unto the king, that I may win the hearts of these my fellow–servants, that I may lead them to believe in my words."  (Book of Mormon | Alma 17:29)

As we read that, one cannot help but pause and wonder how Ammon knew he had such power.  Ammon would later ask king Lamoni -

"...Wilt thou hearken unto my words, if I tell thee by what power I do these things? "  (Book of Mormon | Alma 18:22)

He then explained the source of this power -

"...a portion of that Spirit dwelleth in me, which giveth me knowledge, and also power according to my faith and desires which are in God."  (Book of Mormon | Alma 18:35)

Power then comes through the Holy Ghost by knowledge, faith, and desires to do the will of God.