Rebbi says: It (that separation is implied) is derived from the verse itself, viz. (Ibid. 10) "Go to the people and make them ready today and tomorrow." If immersion itself is meant, let him immerse on the fifth day, so that he will be clean at sunset (for receiving the Torah on the sixth day.) It is, therefore, written "Go to the people (i.e., the married couples) to direct them to separate (for three days).