זהר חדש צא א,
there are 600,000 letters in the Torah, corresponding to the population of Israel, presumably at the time of the Exodus.
I am hardly an expert in the Zohar to say whether or not the idea you asked for is in the Zohar, but this seemed similar enough that it may be what you are referring to.
The relevant quote is:
עַד דְּסָלְקִין ...