Sunan Ibn Majah 31587
In-book no 1754حَدَّثَنَا مُحَمَّدُ بْنُ بَشَّارٍ، حَدَّثَنَا يَحْيَى بْنُ سَعِيدٍ، عَنْ سُفْيَانَ، عَنْ أَبِي إِسْحَاقَ، عَنْ أَبِي الأَحْوَصِ، عَنْ عَبْدِ اللَّهِ، قَالَ طَلاَقُ السُّنَّةِ أَنْ يُطَلِّقَهَا طَاهِرًا مِنْ غَيْرِ جِمَاعٍ .
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)."