Where does the minhag of waiting 5 hours and a mah she hoo come from?
Before getting into where it comes from, it should be clear that this is not the accepted minhag, and it should only be done by people that have such a minhag.
In Divrei Yatziv 7-69 he says that this is what they did in Ropshitz, and he wants to say that perhaps it is because they are relying on the opinions that one does not have to wait until the end of the sixth hour. He does however say that they only did this “l’es hatzorech” only when it was needed.
Divrei Yatziv 7-69, See Kovetz Bais Aharon V’yisroel pg. 71-76 who discusses this extensively. Hakashrus Chap. 10 ftnt.76.