Where is this "Chumros" written?

Question

Where did Rabbi Shternbuch write this opinion on the chumros in the Shulchan Aruch? I've been telling my kids for years that when they learn SA with Mishna Brura they have to understand what is a chumra, minhag, halacha etc. and the differences between them. They think I'm either nuts or an apikores because they think it's all absolute halacha.