The Prophet entered upon her, and a freed slave girl of hers concealed herself. The Prophet asked: "Have her periods begun?" She said: "Yes." He tore a piece of his turban and said: "Cover your head with this."  

أَنَّ النَّبِيَّ ﷺ دَخَلَ عَلَيْهَا فَاخْتَبَأَتْ مَوْلاَةٌ لَهَا فَقَالَ النَّبِيُّ ﷺ حَاضَتْ فَقَالَتْ نَعَمْ فَشَقَّ لَهَا مِنْ عِمَامَتِهِ فَقَالَ اخْتَمِرِي بِهَذَا  

The Prophet said: "Allah does not accept the prayer of a woman who menstruates (i.e., an adult woman) except with a head cover."  

عَنِ النَّبِيِّ ﷺ قَالَ لاَ يَقْبَلُ اللَّهُ صَلاَةَ حَائِضٍ إِلاَّ بِخِمَارٍ