Yahya related to me from Malik from Rabia ibn Abi Abd ar-Rahmanthat on one occasion A'isha, the wife of the Prophet, may Allah blesshim and grant him peace, was sleeping with the Messenger of Allah, mayAllah bless him and grant him peace, in one garment, when suddenly shejumped up sharply. The Messenger of Allah, may Allah bless him andgrant him peace, said to her, "What's the matter with you? Are youlosing blood?", meaning menstruating. She said, "Yes." He said, "Wrapyour waist-wrapper tightly about you, and return to your sleeping-place."
USC-MSA web (English) reference: Book 2, Hadith 96
Arabic reference: Book 2, Hadith 126