Halliday (2008: 13):
I feel it is important to refer to the complementarity in these terms, “language as system” and “language as text”, in order to stress that these are two aspects of one single phenomenon — not two different phenomena, as is implied if you use a simple duality like “language and text”, or “langue and parole”. System and text are one and the same phenomenon; the system is simply the potential that is instantiated in every moment of discourse.