D&C 132

As the Lord teaches the conditions of The New and Everlasting Covenant, He reveals the sin that is unforgiveable saying - 

"The blasphemy against the Holy Ghost, which shall not be forgiven in the world nor out of the world, is in that ye commit murder wherein ye shed innocent blood, and assent unto my death, after ye have received my new and everlasting covenant, saith the Lord God; and he that abideth not this law can in nowise enter into my glory, but shall be damned, saith the Lord."  (Doctrine and Covenants 132:27)

In reference to "commit murder", He explains that He is referring to "innocent blood" or "assent unto my death".

Elder Bruce R. McConkie wrote - 

“The innocent blood is that of Christ; and those who commit blasphemy against the Holy Ghost, which is the unpardonable sin (Matt. 12:31–32), thereby ‘crucify to themselves the Son of God afresh, and put him to an open shame.’ (Heb. 6:6.) They are, in other words, people who would have crucified Christ, having the while a perfect knowledge that he was the Son of God” (Doctrinal New Testament Commentary, 3:345).