Muwatta Imam Malik Book of Purity, Hadith 98

Yahya related to me from Malik that he had heard that Salim ibn Abdullah and Sulayman ibn Yasar were asked whether the husband of a menstruating woman could have sexual intercourse with her when she saw that she was pure but before she had had a ghusl. They said, "No, not until she has had a ghusl."

وَحَدَّثَنِي عَنْ مَالِكٍ، أَنَّهُ بَلَغَهُ أَنَّ سَالِمَ بْنَ عَبْدِ اللَّهِ، وَسُلَيْمَانَ بْنَ يَسَارٍ، سُئِلاَ عَنِ الْحَائِضِ، هَلْ يُصِيبُهَا زَوْجُهَا إِذَا رَأَتِ الطُّهْرَ قَبْلَ أَنْ تَغْتَسِلَ فَقَالاَ لاَ حَتَّى تَغْتَسِلَ .