math reference needed

[gusl]

I'm sure this is a well-known "folk theorem", that feedback loops with product > 1 are unstable, so I should be able to just reference in my paper, without wasting space and worrying about details being wrong. But I don't know where to look.

My deadline is on Friday at 7pm. If you find me a reference before Friday 2pm-ish, I will offer to buy you dinner.

UPDATE: I solved this myself, by noting that if the cycle has length k, then B^k = pi I. (stability is lim_{t->infty} B^t = 0)