I'm not sure which was the first text to use it, but Ackley introduced
it in his intermediate macro text in the 1960s.  (I'm sure it was used
earlier, but I don't know when.)  I think what is important is that for
Ackley, and for other economists at the time,  IS/LM was simply another
way of presenting the model--he preferred the equations and the four
quadrant diagram to IS/LM, and he only included a couple of IS/LM
graphs.  I don't think you can see IS/LM as separate from the single
8-equation model of the macro economy that underlay much of thinking in
the 50s and 60s in macro. I discussed the use of IS/LM in my article on
"The Strange Persistence of the IS/LM Model" in HOPE Vol 36, 2004. 

Dave Colander