(Ibid. 15) "And he said to the people: Be ready in three days": But we did not hear the L rd telling him (in Ibid. 11) that they should separate from their wives! — But "Be ready" — "Be ready" constitutes a gezeirah shavah (identity), viz.: Just as "Be ready" here is linked to separation, so, "Be ready" there.
Where Did Moses Get the Additional Requirement
Mekhilta Tractate Bachodesh 3:17