D&C 46:4; The Articles of Faith, Talmage, p. 175; Answers to Gospel Questions, Smith, 2:88-90; Doctrines of Salvation, Smith, 2:343-344
“The sacrament is a sacred ordinance and should be participated in only when a person is striving sincerely to keep the covenants thereof. However, if a person waits until he is totally free from sin, he will never partake of the sacrament. The sacrament is a source of spiritual power, and compounds the problem of becoming more worthy. However, sins of a serious nature may cause us to lose the privilege of partaking of the sacrament through probation, disfellowshipment, or excommunication. Generally, if a sin is serious enough to warrant not partaking of the sacrament for any length of time, it is probably serious enough to warrant a discussion of that transgression with priesthood leaders.” (Book of Mormon 121-122 Student Manual, pp. 121-122)
“If any of the members are not in good standing; if they have in their hearts any feeling of hatred, envy, or sin of any kind, they should not partake of these emblems. If there are any differences or feelings existing between brethren, these differences should be adjusted before the guilty parties partake… . We should all see that our hearts and hands are clean and pure.” (Joseph Fielding Smith, Doctrines of Salvation, 2:343)