Yahya related to me from Malik from Nafi that Ubaydullah ibn Abdullah ibn Umar sent a question to A'isha asking her, "May a man fondle his wife when she is menstruating?" She replied, "Let her wrap her waist-wrapper around her lower part and then he may fondle her if he wishes."

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّ عُبَيْدَ اللَّهِ بْنَ عَبْدِ اللَّهِ بْنِ عُمَرَ، أَرْسَلَ إِلَى عَائِشَةَ يَسْأَلُهَا هَلْ يُبَاشِرُ الرَّجُلُ امْرَأَتَهُ وَهِيَ حَائِضٌ فَقَالَتْ لِتَشُدَّ إِزَارَهَا عَلَى أَسْفَلِهَا ثُمَّ يُبَاشِرُهَا إِنْ شَاءَ ‏.‏
USC-MSA web (English) reference : Book 2, Hadith 97
Arabic reference : Book 2, Hadith 127