مُوَطَّأُ مَالِكٍ
Muwatta Malik 125
Purity
حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنْ زَيْدِ بْنِ أَسْلَمَ، أَنَّ رَجُلاً، سَأَلَ رَسُولَ اللَّهِ صلى الله عليه وسلم فَقَالَ مَا يَحِلُّ لِي مِنِ امْرَأَتِي وَهِيَ حَائِضٌ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم لِتَشُدَّ عَلَيْهَا إِزَارَهَا ثُمَّ شَأْنَكَ بِأَعْلاَهَا .
Yahya related to me from Malik from Zayd ibn Aslam that a man questioned the Messenger of Allah, (ﷺ), saying, "What is permitted me from my wife when she is menstruating?" The Messenger of Allah, (ﷺ), said, "Let her wrap her waist-wrapper round herself tightly, and then what is above that is your concern."
Arabic/English book reference: Book 2, Hadith 95