Sunan an-Nasa'i, Volume 4, Book 27, Hadith 3424 (DarusSalam) ⓘ
It was narrated that 'Abdullah said: "The Sunnah divorce is to divorce her when she is pure (not menstruating) without having had intercourse with her."
أَخْبَرَنَا عَمْرُو بْنُ عَلِيٍّ، قَالَ حَدَّثَنَا يَحْيَى، عَنْ سُفْيَانَ، عَنْ أَبِي إِسْحَاقَ، عَنْ أَبِي الأَحْوَصِ، عَنْ عَبْدِ اللَّهِ، قَالَ طَلاَقُ السُّنَّةِ أَنْ يُطَلِّقَهَا طَاهِرًا فِي غَيْرِ جِمَاعٍ .
also referenced under: Hadith 3395 (Reference) & Book 27, Hadith 7 (In-Book)