Muwatta Malik #127
Purity
The Arabic includes the chain of narration (sanad) and the collector's commentary; the English translation shows the Prophet's words only.
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 .
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."
About Muwatta Malik
This hadith is #127 from Muwatta Malik, in the Book of Purity.