Wa alaykum salam wa rahmatuLlahi wa barakatuHu,

A partial pure period (even if only a moment) is considered as a complete pure period. Thus, if a woman’s husband divorced her during the pure period, her waiting period ends after two more intervals between menstruations finishes and a third menstruation begins.  (Tuhfat al-Muhtaj v. 8, p. 233)

And Allah knows best.

Shafiifiqh.com Fatwa Dept.