You may know it, I shall allow. But it's not clear what you are asking, equation itself or the source that you can refer. Since the latter covers the first I'm going to answer that: open Gray&Meyer Analysis and Design of Analog Integrated Circuits (The one I have is third edition) and on page 76 you'll find a surprise in equation 1.218. If you have a different edition just open the index and look for subthreshold, it's like googling but ancient.
Man, I'm trying too hard to be funny. I'm bored.