The story was in the “Golem of Prague” book by Gershom Winkler. I just looked it up out of curiosity, and it’s in Reb Yudel Rosenbergs’ נפלאות מהרל which is the origin for pretty much all the Golem stories, but it’s of highly questionable accuracy.
In Prague, there was an abandoned building that people saw strange things around. One merchant passed the building and saw a scary black dog come out, run around him, and go back inside. The man then started having dreams where he was riding a giant dog like a horse in a cavalry charge with other men and they were all barking and howling like dogs. The people in his house noticed that he was sweating, and barking in his sleep. During the day, the man was visibly weakened. After three nights they brought him to the Maharal. The Maharal checked his teffilin and tzitzis and found both to be pasul. He said that this merchant is missing his Malachei Shmira and that’s why the tumah from the building affected him. The man fixed his tzitzis and teffilin, put on an amulet with the pasuk “ולכל בני ישראל לא יחרץ כלב לשונו” and some Kabbaladike letters inscribed on it, and was told to trade beds with Yosele Goiloms for a week. After the week was up, the Maharal instructed Yosele Goilom to burn down the building and no harm came to anyone from the building again.