It has always been my ambition to preach the gospel where Christ was not known, so that I would not be building on someone else's foundation. (NIV)
Thus my ambition has been to preach the Gospel, not where Christ's name has already been known, lest I build on another man's foundation (AMP)
Yea, so have I strived to preach the gospel, not where Christ was named, lest I should build upon another man's foundation: (KJV)