Muwatta Malik 124

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, may Allah bless him and grant him peace, saying, "What is permitted me from my wife when she is menstruating?" The Messenger of Allah, may Allah bless him and grant him peace, said, "Let her wrap her waist-wrapper round herself tightly, and then what is above that is your concern

Reference

Muwatta Malik 124

In-book

Book 2, Hadith 93