Muwatta Malik 125

Yahya related to me from Malik from Rabia ibn Abi Abd ar-Rahman that on one occasion A'isha, the wife of the Prophet, may Allah bless him and grant him peace, was sleeping with the Messenger of Allah, may Allah bless him and grant him peace, in one garment, when suddenly she jumped up sharply. The Messenger of Allah, may Allah bless him and grant him peace, said to her, "What's the matter with you? Are you losing blood?", meaning menstruating. She said, "Yes." He said, "Wrap your waist-wrapper tightly about you, and return to your sleeping- place