I can’t remember specifically, but I’m sure there is a passage somewhere that says something to the effect that if we knew who Joseph Smith was before this life that people would seek to take his life then and there. I have my own theory on this. Is there anything written on the records of The Church that sheds some light on this subject?
It may be that you are remembering a combination of two passages. The first would be by the prophet himself, wherein he said,
“You don’t know me; you never knew my heart. No man knows my history. I cannot tell it: I shall never undertake it. I don’t blame any one for not believing my history. If I had not experienced what I have, I would not have believed it myself. I never did harm any man since I was born in the world. My voice is always for peace.” (Joseph Smith, Discourses of the Prophet Joseph Smith, compiled by Alma P. Burton, p.3)
And the other may have been one of many statements calling for the death of the Prophet, as recorded in Our Heritage A Brief History of the Church of Jesus Christ of Latter-Day Saints, p. 55, as follows–
“On at least 19 different occasions, beginning as early as 1829, Joseph Smith told the Saints that he would probably not leave this life peacefully. While he felt that his enemies would one day take his life, he did not know when. As the spring of 1844 became summer, enemies both within and without the Church worked toward Joseph’s destruction. Thomas Sharp, editor of a nearby newspaper and a leader in Hancock County’s anti-Mormon political party, openly called for the Prophet’s murder. Citizens groups, apostates, and civic leaders conspired to destroy the Church by destroying its prophet.”