The Book of Mormon quotes itself several times. One example is in Moroni 7:26
And as surely as Christ liveth he spake these words unto our fathers, saying: Whatsoever thing ye shall ask the Father in my name, which is good, in faith believing that ye shall receive, behold, it shall be done unto you.
This is a reference to 3 Nephi 18:20
And whatsoever ye shall ask the Father in my name, which is right, believing that ye shall receive, behold it shall be given unto you.
But a few verses later there is another reference to something Jesus taught. In Moroni 7:33
And Christ hath said: If ye will have faith in me ye shall have power to do whatsoever thing is expedient in me.
We don't find this quote anywhere in 3 Nephi. But interestingly, we find another reference to this same statement in Moroni 10:23
And Christ truly said unto our fathers: If ye have faith ye can do all things which are expedient unto me.
Probably, this statement by Jesus was written on the large plates of Nephi, but Mormon chose not to include it in his abridgment. After all, he could not even write "a hundredth part of the things which Jesus did truly teach unto this people" (3 Nephi 26:6).