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