when did masturbation become a sin in the mormon church?

masturbation is not once mentioned in any of the standard works and is not even mentioned in the corpus of LDS General Conference talks (1851-2010) until the 1960's:

http://corpus.byu.edu/gc/

search for "masturbat*" (without quotes)

It was Spencer W. Kimball who first made it into an issue.

The following is supposedly from D. Michael Quinn's book, 'The Mormon Hierarchy : Extensions of Power':

"
18 June, 1870 - First counselor George A. Smith tells Salt Lake School of the Prophets about "the evil of Masturbation" among Utah Mormons. Apostle Lorenzo Snow says that "Plural Marriage would tend to diminish this evil self-pollution," and he believes that "indulgence on the part of men was less in Plural marriage than in Monogamy."
"

great moments in mormon history