It was narrated that Abdullah said:
"Divorce according to the Sunnah means divorcing her when she is pure, ( i.e., not menstruating) and without having had intercourse with her (during that cycle)." (ID)
English reference (ID): Vol. 3, Book 10, Hadith 2020 (ID)
Arabic reference (ID): Book 10, Hadith 2098 (ID)