Yosef Karo, Translation:Shulchan Aruch — Yoreh Deah 287
“ And all the more so a door that is not 10 tefachim high is exempt from the requirement of mezuzah. The practice which is widespread in these areas is that most of the world relies on a single mezuzah that they place on the door to their houses and this is incorrect, and they have nothing [no legal source] to rely on for this. Therefore, every G-d fearing person should fix his house according to the law [so that there will be mezuzot] on all of the doors that require [mezuzot] , as was explained. ”
