3 Nephi 26

Some have wondered why some of the modern revelations and doctrine we have received were not included in the Book of Mormon record.  Mormon answers that question saying -

"...there cannot be written in this book even a hundredth part of the things which Jesus did truly teach unto the people..."  (Book of Mormon | 3 Nephi 26:6)

Adding -

"...these things have I written, which are a lesser part of the things which he taught the people; and I have written them to the intent that they may be brought again unto this people, from the Gentiles, according to the words which Jesus hath spoken.
9 And when they shall have received this, which is expedient that they should have first, to try their faith, and if it shall so be that they shall believe these things then shall the greater things be made manifest unto them."  (Book of Mormon | 3 Nephi 26:8 - 9)

For -

"...I was about to write them, all which were engraven upon the plates of Nephi, but the Lord forbade it, saying: I will try the faith of my people.
12 Therefore I, Mormon, do write the things which have been commanded me of the Lord..."  (Book of Mormon | 3 Nephi 26:11 - 12)

Obviously, the Savior taught much more than was allowed to be written in the Book of Mormon.