The Bible verses that mention the "synagogue of Satan" are Revelation 2:8–9 and Revelation 3:7–9:

"I know the slander on the part of those who say that they are Jews and are not, but are a synagogue of Satan". - Revelation 2:8–9 "I will make those of the synagogue of Satan who say that they are Jews and are not, but are lying—I will make them come and bow down before your feet, and they will learn that I have loved you". - Revelation 3:7–9