that she got a Khul during the time of the Prophet. So the Prophet ordered her - or: she was ordered - that she observe an Iddah of a menstruation."  

أَنَّهَا اخْتَلَعَتْ عَلَى عَهْدِ النَّبِيِّ ﷺ فَأَمَرَهَا النَّبِيُّ ﷺ أَوْ أُمِرَتْ أَنْ تَعْتَدَّ بِحَيْضَةٍ قَالَ وَفِي الْبَابِ عَنِ ابْنِ عَبَّاسٍ  

The wife of Thabit bin Qais was granted a Khul from her husband during the time of the Prophet. So the Prophet ordered her to observe an Iddah of a menstruation.  

أَنَّ امْرَأَةَ ثَابِتِ بْنِ قَيْسٍ اخْتَلَعَتْ مِنْ زَوْجِهَا عَلَى عَهْدِ النَّبِيِّ ﷺ فَأَمَرَهَا النَّبِيُّ ﷺ أَنْ تَعْتَدَّ بِحَيْضَةٍ