Sunan an-Nasa'i, Volume 4, Book 27, Hadith 3423 (DarusSalam) ⓘ

It was narrated from 'Abdullah that he said: "The Sunnah divorce is a divorce issued when she is pure (not menstruating) without having had intercourse with her. If she menstruates and becomes pure again, give her another divorce, and if she menstruates and becomes pure again, give her another divorce, then after that, she should wait for another menstrual cycle." (One of the narrators) Al-A'mash said: "I asked Ibrahim, and he said something similar."

أَخْبَرَنَا مُحَمَّدُ بْنُ يَحْيَى بْنِ أَيُّوبَ، قَالَ حَدَّثَنَا حَفْصُ بْنُ غِيَاثٍ، قَالَ حَدَّثَنَا الأَعْمَشُ، عَنْ أَبِي إِسْحَاقَ، عَنْ أَبِي الأَحْوَصِ، عَنْ عَبْدِ اللَّهِ، أَنَّهُ قَالَ طَلاَقُ السُّنَّةِ تَطْلِيقَةٌ وَهِيَ طَاهِرٌ فِي غَيْرِ جِمَاعٍ فَإِذَا حَاضَتْ وَطَهُرَتْ طَلَّقَهَا أُخْرَى فَإِذَا حَاضَتْ وَطَهُرَتْ طَلَّقَهَا أُخْرَى ثُمَّ تَعْتَدُّ بَعْدَ ذَلِكَ بِحَيْضَةٍ ‏.‏ قَالَ الأَعْمَشُ سَأَلْتُ إِبْرَاهِيمَ فَقَالَ مِثْلَ ذَلِكَ ‏.‏