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)." (IT)
English reference (IT): Vol. 3, Book 10, Hadith 2020 (IT)
Arabic reference (IT): Book 10, Hadith 2098 (IT)