(Ibid. 15) "Do not draw near to a woman": From here it was ruled that if a woman ejects semen on the third day (after intercourse) she is pure, as is implied by the Sinai directive. These are the words of R. Elazar b. Azaryah.
Moses Delivered This Instruction Three Days Before the Torah Was Given
Mekhilta Tractate Bachodesh 3:19