I believe that the original question dealt with who was the first to posit the "parallel to a line through a point" form of Euclid's postulate. Although some have claimed that Proclus hints at it in his discussion of Euclid I.31, I don't believe it is that explicit. Calinger claims that Playfair writes :
" Given a line l and a point P not on l, there exists one and only one line m containing P and no points of l."
It is contained in section 11.1 of Elements of Geometry written in 1795. I have a copy of Davies Legendre dated 1844 which gives the following axiom on page 13 :
" 12. Through the same point, only one straight line can be drawn which shall be parallel to a given line."
Considering that Legendre published his Elements de Geometrie in 1794, is it possible that the first explicit modern statement is his? Did he lose credit for a discovery once again?