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