Yahya related to me from Malik that he had heard that Salim ibnAbdullah and Sulayman ibn Yasar were asked whether the husband of amenstruating woman could have sexual intercourse with her when she sawthat she was pure but before she had had a ghusl. They said, "No, notuntil she has had a ghusl."
USC-MSA web (English) reference: Book 2, Hadith 98
Arabic reference: Book 2, Hadith 128