here you go
From the Book Teachings of the Prophet Joseph Smith Page 358:
All sins shall be forgiven, except the sin against the Holy Ghost; for Jesus will save all except the sons of perdition. What must a man do to commit the unpardonable sin? He must receive the Holy Ghost, have the heavens opened unto him, and know God, and then sin against Him. After a man has sinned against the Holy Ghost, there is no repentance for him.12 He has got to say that the sun does not shine while he sees it; he has got to deny Jesus Christ when the heavens have been opened unto him, and to deny the plan of salvation with his eyes open to the truth of it; and from that time he begins to be an enemy. This is the case with many apostates of the Church of Jesus Christ of Latter-day Saints.
The quote be SWK is from MoF page 123 and I have edited out commentary from the source I got it from. My apologies. In my rush I did not notice it was not all from him:
"The sin against the Holy Ghost requires such knowledge that it is manifestly impossible for the rank and file to commit such a sin" (The Miracle o Forgiveness, p.123)"
“Lesson 20: The Kingdoms of Glory,” Doctrine and Covenants and Church History Gospel Doctrine Teacher’s Manual, 110
2. Perdition
Teach and discuss D&C 76:25–39, 44–49. Begin by writing Perdition on the chalkboard. Explain that the word perdition refers to a state of loss and destruction rather than a kingdom of glory. Those who experience this are called “sons of perdition” because they follow Satan, who is called Perdition (D&C 76:25–26, 31–32). Be sure to keep this discussion brief, avoiding speculation and leaving sufficient time to discuss the celestial kingdom later in the lesson. As with other subjects, focus only on what has been taught in the scriptures and by latter-day prophets.
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• When Lucifer rebelled in the Council in Heaven, he was thrust down to earth (Revelation 12:7–9; D&C 29:36–37; 76:25–28; Moses 4:1–3). What did he begin to do when he was cast out? (See D&C 76:29; Moses 4:4.) What can we do to win the war against Satan in our lives? (For some answers to this question, see 1 Nephi 14:14; D&C 10:5; 27:15–18.)
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• What did the Lord reveal about the suffering of the sons of perdition? (See D&C 76:32–34, 36–38, 44–49.) Why are sons of perdition condemned to experience such great suffering? (See D&C 76:30–31, 35. For an explanation of what it means to deny the Holy Ghost, see the following quotation.)
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The Prophet Joseph Smith said: “What must a man do to commit the unpardonable sin? He must receive the Holy Ghost, have the heavens opened unto him, and know God, and then sin against Him. After a man has sinned against the Holy Ghost, there is no repentance for him. He has got to say that the sun does not shine while he sees it; he has got to deny Jesus Christ when the heavens have been opened unto him, and to deny the plan of salvation with his eyes open to the truth of it; and from that time he begins to be an enemy” (Teachings of the Prophet Joseph Smith, sel. Joseph Fielding Smith [1976], 358).
Cecil O. Samuelson Jr., “Words of Jesus: Forgiveness,” Ensign, Feb 2003, 48
All Sins but One
The Savior was very clear that, conditioned on repentance, all of our sins can be forgiven through His sacred and atoning sacrifice except for what He called “blasphemy against the Holy Ghost” (Matt. 12:31; see also Mark 3:28–29). The Prophet Joseph Smith taught on this subject: “Jesus will save all except the sons of perdition. What must a man do to commit the unpardonable sin? He must receive the Holy Ghost, have the heavens opened unto him, and know God, and then sin against Him.” Teachings of the Prophet Joseph Smith, sel. Joseph Fielding Smith (1976), 358.