Where in The Book of Mormon does it say that Joseph Smith told the truth?

Actually, the Book of Mormon indicates that Joseph Smith did not tell the truth. Follow me carefully.

Another answer included here makes reference to the witnesses of the Book of Mormon. Let's go from there.

The Book of Mormon speaks of these.

2 Nephi 27:12 Wherefore, at that day when the book shall be delivered unto the man of whom I have spoken, the book shall be hid from the eyes of the world, that the eyes of none shall behold it save it be that three witnesses shall behold it, by the power of God, besides him to whom the book shall be delivered; and they shall testify to the truth of the book and the things therein.

LDSaints like to point our that they testified "to the truth of the book". But they always neglect the second part, "and the things therein". This is very important. Because they ignore the second part, they completely misuse Moroni's promise found in Moroni 10.

3. Behold, I would exhort you that when ye shall read these things, if it be wisdom in God that ye should read them, that ye would remember how merciful the Lord hath been unto the children of men, from the creation of Adam even down until the time that ye shall receive these things, and ponder it in your hearts.
4. And when ye shall receive these things, I would exhort you that ye would ask God, the Eternal Father, in the name of Christ, if these things are not true; and if ye shall ask with a sincere heart, with real intent, having faith in Christ, he will manifest the truth of it unto you, by the power of the Holy Ghost.
5. And by the power of the Holy Ghost ye may know the truth of all things.

LDSaints will tell you that the Holy Ghost will tell you that the Book is true. They go on to tell you that if the Book is true, that is, that the gold plates existed and so Joseph was a prophet. This is a major error. Let me edit somewhat Moroni's Promise in light of the above quoted verse.

3. Behold, I would exhort you that when ye shall read these things [therein], if it be wisdom in God that ye should read them, that ye would remember how merciful the Lord hath been unto the children of men, from the creation of Adam even down until the time that ye shall receive these things [therein], and ponder it in your hearts.
4. And when ye shall receive these things [therein], I would exhort you that ye would ask God, the Eternal Father, in the name of Christ, if these things [therein] are not true; and if ye shall ask with a sincere heart, with real intent, having faith in Christ, he will manifest the truth of it unto you, by the power of the Holy Ghost.
5. And by the power of the Holy Ghost ye may know the truth of all things [therein].

But we know that Joseph Smith taught things contradictory to the "things therein". Oddly LDSaints accept the Book as true, but reject much of the "things therein". Their major doctrines, those which set them apart from Christianity, are all contradictory to the "things therein" almost to the point where one can conclude that the "things therein" were specifically included in the Book of Mormon for the purpose of warning the LDSaints! This is especially true because it was wisdom in God that [they, the LDSaints] should read them!! Christians believe, for the most part, the “things therein” and so they don’t especially need the testimony of the Book of Mormon. Consider “when ye shall read these things, if it be wisdom in God that ye should read them”. It follows that it is wisdom in God that some don’t read them, probably because they already believe the “things therein”. I associate with Christians all the time who believe the “things therein” without having read the Book of Mormon! It is LDSaints that need to make “course corrections” according to the “things therein”.

Repentance after death, polygamy, various kingdoms of glory, temples, etc find condemnation in the Book of Mormon. Hence, we see that in the Book of Mormon, the things therein", clearly indicate that Joseph was not a truth-teller.

How do LDSaints deal with the contradictions between the "things therein" and LDS doctrine? They do it in many ways. Of note is an astonishing statement of one of Quora's top LDS scholars.

"Much of what was recorded by those [Book of Mormon] prophets was based on an Old Testament misunderstanding of God and his Son and ancient Israelite views of heaven, hell, and God’s plan of salvation.

"Many of these misunderstandings were corrected later in the Book of Mormon when the resurrected Lord appeared to the Nephites. One concept that doesn’t seem to have been clarified is what is revealed in Doctrine and Covenants 76 and 88 regarding degrees of glory in heaven. The ancient Israelite/Jewish/Christian heaven-hell dichotomy still seems to be reflected in the Book of Mormon.

"Other similar teachings such as an understanding of the Spirit World, the pre-mortal existence of man, baptism by proxy for the dead, the unity of the Godhead, and many modern doctrines, are not discussed. Some see these as conflicting teachings, others see them as reflecting a more limited and incomplete view of the gospel in its fulness".

It should be pointed out that "teachings such as an understanding of the Spirit World, the pre-mortal existence of man, baptism by proxy for the dead, the unity of the Godhead, and many modern doctrines", are actually discussed in the Book of Mormon but, because the teachings of the Book of Mormon contradict LDS doctrine, “these things therein” must be ignored or explained into meaninglessness. Furthermore, the Prophets didn't write according to “misunderstandings”. Indeed, they proclaimed that the very words they spoke were given them from God Himself!

Moroni 10

27. And I exhort you to remember these things; for the time speedily cometh that ye shall know that I lie not, for ye shall see me at the bar of God; and the Lord God will say unto you: Did I not declare my words unto you, which were written by this man, like as one crying from the dead, yea, even as one speaking out of the dust?
28. I declare these things unto the fulfilling of the prophecies. And behold, they shall proceed forth out of the mouth of the everlasting God; and his word shall hiss forth from generation to generation.
29. And God shall show unto you, that that which I have written is true.

2 Nephi 33:10–1 ...And if ye shall believe in Christ ye will believe in these words, for they are the words of Christ, and he hath given them unto me....And if they are not the words of Christ, judge ye--for Christ will show unto you, with power and great glory, that they are his words, at the last day; and you and I shall stand face to face before his bar; and ye shall know that I have been commanded of him to write these things, notwithstanding my weakness.

We see that the Nephite Prophets were given the words they wrote and that they were not fumbling around jotting down "Old Testament misunderstanding[s] of God and his Son and ancient Israelite views of heaven, hell, and God’s plan of salvation". True prophets of God do not present as truth “personal, though well-considered, opinions”! False ones do!

Because Joseph and Sidney decided to imagine their own doctrine while treating lightly "the things" found in the Book of Mormon, their minds had been darkened and the whole church was under condemnation.

Doctrine and Covenants 84

54. And your minds in times past have been darkened because of unbelief, and because you have treated lightly the things you have received--
55. Which vanity and unbelief have brought the whole church under condemnation.
56. And this condemnation resteth upon the children of Zion, even all.
57. And they shall remain under this condemnation until they repent and remember the new covenant, even the Book of Mormon and the former commandments which I have given them, not only to say, but to do according to that which I have written--

The LDSaints never repented of taking the Book of Mormon, "the new covenant", lightly and so their minds have been darkened and remain under the "condemnation". In this state of darkened minds, all manner of false doctrines were presented as the Word of the Lord as delivered by Joseph Smith and those "Living Oracles" who followed.

Before citing the above verses from Section 84, Ezra T. Benson said in 1986 in conference talk called "The Book of Mormon—Keystone of Our Religion":

"Let us not remain under condemnation, with its scourge and judgment, by treating lightly this great and marvelous gift the Lord has given to us. Rather, let us win the promises associated with treasuring it up in our hearts".

We see that in 1986 the Church was still under the condemnation of darkened minds. So what of all those doctrines developed by Joseph/Sidney and others in this period of darkened minds? Well, we can see they are not the truth wherever they contradicted the Book of Mormon and Bible.

Book of Commandments (1833), Chapter 44, Page 91
13 And again, the elders, priests, and teachers of this church, shall teach the scriptures which are in the bible, and the book of Mormon, in the which is the fulness of the gospel;