Ether 12

It is while writing an abridgement of the record of the Jaredites, that Moroni provides one of the greatest discourses on faith.  He begins that -

"...whoso believeth in God might with surety hope for a better world, yea, even a place at the right hand of God, which hope cometh of faith, maketh an anchor to the souls of men, which would make them sure and steadfast, always abounding in good works, being led to glorify God."  (Book of Mormon  Ether 12:4)

He then follows with -

"...I would show unto the world that faith is things which are hoped for and not seen; wherefore, dispute not because ye see not, for ye receive no witness until after the trial of your faith."  (Book of Mormon  Ether 12:6)

To explain that, nothing of  good was ever given or granted without faith in Chirst giving several examples -

"For it was by faith that Christ showed himself unto our fathers, after he had risen from the dead; and he showed not himself unto them until after they had faith in him; wherefore, it must needs be that some had faith in him, for he showed himself not unto the world."  (Book of Mormon   Ether 12:7)

Here recalls the faith of many prophets that brought great miracles, also teaching the doctrine that -

"...neither at any time hath any wrought miracles until after their faith; wherefore they first believed in the Son of God."  (Book of Mormon  Ether 12:18)

Even of the coming forth of the Book of Mormon -

"...it is by faith that my fathers have obtained the promise that these things should come unto their brethren through the Gentiles; therefore the Lord hath commanded me, yea, even Jesus Christ."  (Book of Mormon  Ether 12:22)
And then Moroni teaches even of the Savior  -

"For if there be no faith among the children of men God can do no miracle among them..."  (Book of Mormon   Ether 12:12)

Even that faith was necessary for God to grant the greatest miracle and gift of all...the atonement -

"...because of the faith of men he has shown himself unto the world, and glorified the name of the Father, and prepared a way that thereby others might be partakers of the heavenly gift, that they might hope for those things which they have not seen.
9 Wherefore, ye may also have hope, and be partakers of the gift, if ye will but have faith."  (Book of Mormon  Ether 12:8 - 9)

Therefore -

"...I will show unto them that faith, hope and charity bringeth unto me—the fountain of all righteousness."
(Book of Mormon   Ether 12:28)

In summary, if we desire a miracle of the Lord, we must demonstrate faith in him through fasting, prayer, obedience...being a demonstration of faith.

I would add on important, but related verse -

"...if men come unto me I will show unto them their weakness. I give unto men weakness that they may be humble; and my grace is sufficient for all men that humble themselves before me; for if they humble themselves before me, and have faith in me, then will I make weak things become strong unto them."  (Book of Mormon   Ether 12:27)