The Hebrew Bible ("Old Testament" to Christians) does not actually say that, and in fact mentions people using the name YHVH (however it was actually pronounced) in common speech. (See, e.g., Ruth 2:4). In Second Temple times (ca. 500 BCE- 70 CE), the custom became not to pronounce that name in ordinary speech; at first, it was used only by priests at services in the Temple, and eventually (according to the Talmud) only by the High Priest at the service on Yom Kippur.